It seems as though the required field validator is not working in a case where the data an IEnumerable of POCO's. Perhaps Radzen is seeing a default guid value of {00000000-0000-0000-0000-000000000000} instead of null?
<RadzenDropDown @bind-Value=@(subscription.TriggerId) Name="Trigger" AllowClear="false"
TValue="Guid" AllowFiltering="false" ValueProperty="Id" TextProperty="FullName"
Context="trigger" Data=@triggers />
<br />
<RadzenRequiredValidator Popup="false" Component="Trigger" Text="Trigger is required"
Style="position: absolute" />