Is the plugin you use by any chance Valhalla Shimmer? Because that's the only plugin I know that exhibits exactly this behaviour.
(For the reference: OpenMPT has auto-suspend code for plugins but it's disabled by default and can currently not be enabled for newly created files - so it's not OpenMPT that cuts the plugin off in this case)

All Valhalla plugins I tested exhibit it(UberMod, VintageVerb). Now I am properly testing other plugins that I use more rarely, and yes, my impression that it affected all of them was wrong. embarrassment

After further investigation, Valhalla Shimmer sets the "effFlagsNoSoundInStop" flag, which tells the host that it will not produce any output if the input is silence. So this particular plugin is buggy, as it allows us to remove unnecessary render calls. I've notified the plugin author. Of course it's possible that other reverb plugins by other authors also set this flag, but it would seem weird to me if so many plugin authors would set this flag that is supported by almost no hosts.

Thank you!

I talked to Sean from ValhallaDSP and he said that he confirmed that he's using an old versions of JUCE (which is the source of this bug) and cannot upgrade it for now. Until the user can manually toggle this setting, I have completely disabled silence detection for VST plugins. You can grab a r7553 or later from https://buildbot.openmpt.org/ to test this.