This appears to be a new issue. Unlike the sustain pedal issue, this is present on both Windows and macOS. It is unrelated to my workaround for the sustain pedal issue. When I enter a note with the mouse in the piano roll, the note ID for the note on and off messages do not match. Once again this behaviour can be demonstrated not just with Serum 2 but also with the note expression synth example plug-in from the VST3 SDK. @Arne_Scheffler you may wish to look into this.
The sustain pedal issue on Windows does now seem fixed, so I will remove my workaround from the next build of Serum 2.