Yep, that fixed it and all works as expected now. Yes, it seems that in 4.5.x and earlier it wasn't necessary for that to be checked always.
I think I was confused about the purpose of "update on preset change", as in when I change preset, will that button's current state get updated in the stored preset? Or perhaps the inverse action: when checked, update the IA across presets to match.
A little ambiguous of a label, maybe. But likely more of a case of me not RTFM closely enough. Cheers.
It's been a tricky setting to name, the name has changed several times over the years, and it's still not 100% correct, I feel.
The actual meaning in this context is "when a preset is loaded, update this button's state with whatever state is stored in the preset".