I had a response from Suffolk Archives on the issue with the Butley registers.
They are aware of the problem and have raised it with Ancestry. Once resolved we should be able to see the registers they have for Butley, which do indeed only date from 1785 for baptism/burial and 1792 for marriages (the latter are already viewable).
Although it was not explicitly stated in their response, from what I can find out it seems that the original PRs for Butley prior to 1785 do not exist. I have looked extensively online and checked Phillimore and they all list a start date of 1785.
The FindMyPast transcriptions must have come from the BTs. Suffolk Archives advised these have been digitised too and they plan to add them to the collection, but they do not have a live date yet.
The BTs for Butley (based on TNA entry) cover 1692-1698, 1705, 1708, 1716, 1718, 1723-1725 1727-1729, 1731, 1733-1736, 1738-1747, 1749-1752, 1754-1771 (and presumably some later ones too). Not ideal but hopefully I will still be able to make some further progress despite the multiple gaps.
Reminds me of when I discovered, some years ago now, that the registers for Monkwearmouth in Durham had gone up in flames in the rector's house in 1790, leaving virtually nothing behind except patchy BT coverage. Kind of galling to know all this information once existed and now it's just gone!