I spotted that MAX1B had been changed from 239 to 237 in 4.1, whereas I8B and F10 are now handled as "impossible length sequences" to maximise compatibility with pre-4.1 serialize()d things.
It still works, but encodes 238 and 239 longer than it needs to.
All very insignificant, I know, but MAX1B can be put back to 239 and it should all still work.