You've patiently tried suggestions, and you've said this happened to you on earlier builds. At this point, I recommend you send an email to support@pgmusic.com and reference this thread.

As Dan mentioned, there have been couple of times a weird problem affected only one person. One example I recall is for only those people running a certain AMD CPU. Some combination of your software and hardware may be doing this just to you.

I do particularly wonder about this statement: "This kind of thing has been typical when using the default synthesizer. If I put a separate synth on individual tracks, they stay put. "

BIAB 2025 Win Audiophile. Software: Studio One 7 Pro, Swam horns, Acoustica-7, Notion 6, Song Master Pro, Win 11 Home. Hardware: Intel i9, 32 Gb; Roland Integra-7, Presonus 192 & Faderport 8, Royer 121, Adam Sub8 & Neumann 120 monitors.