It appears the UP/DOWN arrow functionality on an open popup is no longer working. I get no response with the down arrow and a somewhat random selection with the up arrow. If I remember correctly, I believe this was working in 4.x, but I tried reverting back from 5.1.4 and I'm getting the same issue after clearing cache and reloading the js file. See demo site "Custom filtering with LoadData event" for an example.
https://blazor.radzen.com/dropdown-datagrid?theme=material3#custom-filtering-with-loaddata