I am doing CRUD operations on a Master Detail models Team and TeamMember using modal popup. I followed

From the shared code, I have managed to build above output therefore, the expected behaviour might be while clicking on details button on the team's list which is the index page should display the details list of all the team member of that particular team. If that is the business scenario, then I would say, your approach were wrong and you are doing mess of course here thus, follow the steps below which will help to resolve your issue.

Database:

Note: Models would be same as yours. So not repeating that part.

Controller:

public class TeamController : Controller
    {
        private readonly ApplicationDbContext _context;
        private readonly IWebHostEnvironment _environment;


        public TeamController(IWebHostEnvironment environment, ApplicationDbContext context)
        {
            _environment = environment;
            _context = context;
        }
        public IActionResult Index()
        {
            var teamList = _context.Teams.Include(a => a.TeamMembers).ToList();
            return View(teamList);
        }
        
    }

Note: You are doing the mess here as you are using another details action for the _DetailTeamPartialView which is complete Irelevant here. Only Index action is capable here what you are trying to achieve. Just need to use Include(a => a.TeamMembers) which the action.

View Of Index()

@model IEnumerable<DotNet6MVCWebApp.Models.Team>

@{
    ViewData["Title"] = "Index";
}

<table >
    <thead>
        <tr>
            <th>
                @Html.DisplayNameFor(model => model.TeamId)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.TeamName)
            </th>
            <th>
                Member Details
            </th>
        </tr>
    </thead>
    <tbody>
        @foreach (var item in Model)
        {
            <tr>
                <td>
                    @Html.DisplayFor(modelItem => item.TeamId)
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.TeamName)
                </td>
                <td>
                    <button  data-bs-toggle="modal" data-bs-target="@("#DetailTeam-" item.TeamId)">Detail</button>
                    @await Html.PartialAsync("_DetailTeamPartialView", item)
                </td>
     
            </tr>
           
           
        }
    </tbody>
</table>

_DetailTeamPartialView

Though, your _DetailTeamPartialView is alrigh and no vital issue has not seen,

@model DotNet6MVCWebApp.Models.Team

@{
    ViewData["Title"] = "_DetailTeamPartialView";
}
<div  role="dialog" tabindex="-1" id="@("DetailTeam-" Model.TeamId)" aria-labelledby="DetailTeamLabel" aria-hidden="true" data-backdrop="static" data-keyboard="false">
    <div  role="document">
        <div >
            <div >
                <h5 >Detail Team</h5>
                <button type="button"  data-dismiss="modal" aria-label="Close" onclick="javascript:window.location.reload()">
                    <span aria-hidden="true">&times;</span>
                </button>
            </div>
            <div>

                <dl >
                    <div style="margin-left:10px">
                        <dt >
                            @Html.DisplayNameFor(model => model.TeamName)
                        </dt>
                        <dd >
                            @Html.DisplayFor(model => model.TeamName)
                        </dd>
                        .......

                        <dt >
                            @Html.DisplayNameFor(model => model.TeamMembers)
                        </dt>
                        <dd >
                            <table >
                                <thead>
                                    <tr>
                                        <th>Member name</th>
                                        <th>Birth date</th>
                                    </tr>
                                </thead>
                                <tbody>
                                    @for (int i = 0; i < Model.TeamMembers.Count; i  )
                                    {
                                        <tr>
                                            <td>
                                                <input asp-for="@Model.TeamMembers[i].MemberName"  readonly />
                                            </td>
                                            <td>
                                                <input asp-for="@Model.TeamMembers[i].BirthDate"  readonly />
                                            </td>

                                        </tr>
                                    }

                                </tbody>
                            </table>
                        </dd>
                    </div>

                </dl>

            </div>
        </div>
    </div>
</div>

Output

Takeaways:

As you are using HTML Tag helper class that is @await Html.PartialAsync("_DetailTeamPartialView", item) so you don't need to use below Detail action on your controller. Index action would handled the _DetailTeamPartialView autometically.

public IActionResult Detail(int id)
    {
        Team team = _dbcontext.Team
            .Include(a => a.TeamMembers)
            .Where(e => e.TeamId == id).FirstOrDefault();
        return PartialView("_DetailTeamPartialView", team);
    }