To fix this, there are 3 options (pick 1):
1. Cherry-pick *both* 5650c067 and b8b71a25
2. Follow the instructions in [5]
3. Apply the appropriate patch from [6]
Option (3) will the easiest, but it will only work if your LuaTeX source
very closely corresponds to the source in an upstream TL release.
Otherwise, you'll need to do option (2). Option (1) is the same as
option (2), except I've already gone to the trouble of reducing the
patch to the bare minimum.
Many thanks for your help! I'll try to check out, which option to use.