I think I've found how to reproduce this issue.
Open the process buffer with:
edit ("+b Build (.process)");
Don't have checked in the build window: "Send Output to Editor Window"
Do a bunch of builds while you have the process buffer as an editor tab. Perhaps even do Ctrl+Home in the .process editor tab before a build.
Do this enough times and you will like see the <SOH> in the build output.
It is still happening for me in 27.0.1 on Linux x64.