TL;DR this testcase:
<!DOCTYPE html>
<div style="display: flex; width: 100px;">
<div style="aspect-ratio: 1; max-height: 50px; flex: 1; background: green;"></div>
</div>
Should the rect be 100x50 (Firefox/WebKit) or 50x50 (Blink temporarily).
Basically all browsers were previously ignoring the transferred max constraint in this case. Blink shipped an (unintentional) behaviour change and received a bug report about it.
I'm going to change Blink back for the moment, but this case seems un-addressed in the spec currently.
@tabatkins @fantasai