Finally, the problem was in this part of the code:

 <BootstrapSelect
            TItem="Models.DropdownItem<String>"
            Data="@positionsItems"
            @bind-Value="model!.Position"
            TextField="@((item) => item.Value)"
            ValueField="@((item) => item.Key.ToString())"
            TType="Biwenger.Enums.Positions">
        </BootstrapSelect>

It seems that BootsatrpSelect doesn't work well with Generic types or Enums. Binded it to a int variable and the problem dissapears.

Thank you all for your feedback