I'm not aware of any Ubuntu-specifics that might cause this. I would suggest:
1. Testing the 46.0 upstream tag to see if it's just that Ubuntu hasn't released the fix yet; and
2. If 46.0 upstream did not have the bug (meaning it can't be bisected), then report an upstream bug anyway since they have more expertise and eyeballs available.
I'm not aware of any Ubuntu-specifics that might cause this. I would suggest:
1. Testing the 46.0 upstream tag to see if it's just that Ubuntu hasn't released the fix yet; and
2. If 46.0 upstream did not have the bug (meaning it can't be bisected), then report an upstream bug anyway since they have more expertise and eyeballs available.