Someone did that: [v3 Update] Native Apple Touch Bar Icon Pack for BTT
I can't include them in BTT for legal reasons.
The issue is very weird though. It seems like the system doesn't find the standard icons? How did you go about downgrading to 10.15.3? Do the icons still show correctly in the system Touch Bar?
You can also downgrade to any previous BTT version via https://folivora.ai/releases but I'm not sure whether it's really the alpha that causes the issue