AU automation lanes are overwritten in Reaper after reopening a project

If you create automation curves in latch mode, save the project, and reopen it, the automation curves will be overwritten during playback. The problem occurs in Reaper for AU. With VST and VST3 everything is fine. See video.

Tested with JUCE GainPlugin in Reaper v6.56 and JUCE 6.1.6.

Is it possible that this is a JUCE problem? Does anyone know more details?

Thanks for reporting. I’ve added a fix for this issue on the juce7 branch:

I think the root cause of the issue is the same as for the issue discussed here:

We avoided making this change previously because we were worried about potential breakage in other hosts. However, now we know that two popular hosts are broken with the current behaviour, there’s a stronger case for making the suggested change. The documentation for kAudioUnitProperty_ClassInfo (used to set the state of the AU) suggests that the host itself should be responsible for fetching any changed parameter values after a new state is loaded. That is, if this change does cause issues for any hosts, I think that this would indicate a problem with the host, rather than with the plugin.

Haven’t played with your fix, but immediately wondering:
Would this change mean that calling up a preset during playback will NOT put the parameters into auto-write? What about when parked and calling up a preset? It’s very common to make a selection, call up the preset for the scene, then hit play to write automation.