This is a known issue and is already reported and in our backlog:
The latest official release of the Telerik.UI.for.AspNet.Core targets the .Net Core 3.0 RC. We currently worked on the full support of the officially released .Net Core 3.0. and this problem has already been resolved. You could expect to obtain the fix with the next official R3 2019 SP1 which is scheduled for this week.
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