Thanks for the reminder, @webwrotter – I probably should have reported back earlier. I’ve actually been talking to ACF devs about this, who have been fantastic at researching and chatting about it. (Mad props to Liam G, especially.)
It turns out that ACF is behaving exactly like native WordPress core blocks do at the moment. While I don’t love how core handles it, I wouldn’t expect ACF to do things any differently.
This issue covers most of it: https://github.com/WordPress/gutenberg/issues/7342
But basically, WP core sees default values to be used at runtime, rather than when the block is inserted. Feel free to voice your opinion for change in the Github issue along with mine and some others
