Interesting! As that's the nightly version, it's also possible it has some unresolved bugs etc, so it's not something I'll really be looking into solving unless it makes it into the public 8.4 release. Though, engine updates generally should not retroactively change the behaviour of shaders, so it would likely be an engine regression of some kind in that case. If you can reproduce the behaviour change from 8.3 to 8.4 in a basic project, it might be worth a bug report in the Ren'Py GitHub.