Hi,
I just checked it with Mixcraft. And the problem here is, that the host calls the IEditController instance of AGain on a secondary thread, not on the main thread. This is a host problem and I expect that the other hosts have the same issue. A host is only allowed to call the edit controller of a VST3 plug-in from the main thread as described in the VST3 documentation (see “Edit Controller Call Sequence”).
With VSTHost from Herman Seib I was not able to reproduce this issue, and I don’t have Tracktion Waveform installed to check.
Cheers,
Arne