The 64KB limit isn't on the exported file size, it's the maximum size of a compiled module.
If your module is less than 10K lines, if can be compiled.
A heavy but likely still healthy module would be 1K lines, tops - that seems to hover around 40KB when exported to a text file; 64KB doesn't strike me as completely indecent, although it is definitely above 1K lines of code, and thus could likely use some tweaks.
If your modules are named e.g.
Utilities, verify how cohesive their members are - is it all related to the same functionality? Or does it feel like random functions were dumped in there?
Look for repeated code, refactor it. Extract methods, parameterize them, watch your module melt away, while retaining all its functionality.
The internal storage mechanics for VBA code, haven't changed in 20 years - I don't see a reason for that to have changed recently, especially since VBA is pretty much frozen now, and changing anything in the storage mechanics would break millions of things everywhere.
But, it's not impossible that something in O365 was changed recently (are you using an Insider build?), and something broke and your workbook somehow got corrupted.... but if your module is slightly above 64KB of text source code, it's very unlikely to be related: the compiled code would be much smaller than that... assuming the project compiles.