Sorry for the bump, but I had an NTSC 800XL from ebay with the issue from the first post. Swapped the 4050 with one from a PAL 600XL (with U1MB installed) and the 800XL is all good.
However, put the dodgy 4050 in the 600XL, and it's all good there too!
Using contact cleaner and swapping the chips over a few times consistently brought the same results: both were good in the PAL 600Xl, only one worked correctly in the 800XL.
Should I be expecting different faults in the 600XL to appear with dodgy 4050? (I've ordered a couple anyway).