I am also having a similar issue, but not upgrading from 4.8 to 4.9 (just using 4.8).
@TJ I realise this isn’t a lot to go on, but it is very hard from our end to work out a repro - it seems to be caused by inherited blueprints, (mine at least a blueprint structure) and events, but beyond that, fact that it compiles if you press compile suggests there isn’t some obviously incorrect setup.
If you want to pull my project and investigate, DM me, but it certainly isn’t a minimal project by any stretch of imagination.