I've just made the following builds:
Geany 1.38 built with the latest GTK and other dependencies, including some extra fixes which I plan to release as the official macOS build on the Downloads page unless there are some problems with it: https://download.geany.org/snapshots/geany-1.38_osx-5.dmg https://download.geany.org/snapshots/geany-1.38_osx_arm64-2.dmg
Like the above but Geany and plugins are from current git master: https://download.geany.org/snapshots/geany-1.38_osx.dmg https://download.geany.org/snapshots/geany-1.38_osx_arm64.dmg
I decided to "fix" this by trading one bug for another. Instead of the `\0` in the copy/pasted text that indicates block copy I use `\1` which is preserved by macOS. This will include the `\1` when the block-copied text is pasted outside Geany but I don't think people commonly use block-copy to paste it to other applications (it doesn't work as a block-copy in this case anyway).
I just created a simple patch doing this on top of Geany sources that is applied automatically during macOS binary build - it's really just an ugly workaround which I don't want to submit upstream.