Hot Reloading fails in 4.21 if a project code class that derives from a plugin code class is modified. The Editor appears to be looking for a corresponding Hot Reload .dll for the plugin, even though the plugin is not being updated.
REGRESSION:
Yes. A Hot Reload in this instance would complete successfully in 4.20.3
RESULT:
The Hot Reload fails with an error similar to the following: Missing import: UE4Editor-MyPlugin-3343.dll
EXPECTED:
The Hot Reload completes successfully.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-65127 in the post.