I have several issues with the Missing font tool :
A) when several fonts are missing, clicking one and then another will make a group selection with no apparent way to unselect other than re-opening the file
B) when choosing to replace a font, it seems the font updates when selecting a font family (without clicking on OK - which appears as a blank button by the way) causing an inconsistent font replacement when selecting a font style and then clicking on OK (some text may remain "regular" when I selected "bold" for example while some others do update correctly).
- one showing the selection issue (as well as missing label on "OK" button)
- one showing the issue when replacing fonts, you'll see only the "regular" one is working as intended as the texts update when selecting the font family but won't apply again when choosing a different font style and clicking OK.
The visual bugs appear consistently on every files (wether they're saved locally or on the cloud). As for the replacing issue I've only tried it with Helvetica Neue LT Std to Helvetica Neue, can do further testing if necessary!