I have exactly same issue!
For example I have various projects with FA installed using Ariel / Ariel Bold Rollback and now every single site project containing FA changed from Sans-Serif to Sans/Times Roman IF project is saved AFTER January 18th, its very notifiable across multiple project.
This ONLY affects website projects saved after January 18. I tested Sifari Firefox Chrome IE Edge browsers on multiple devices, it looks to be a webflow issue.
Sites before January 18th are not affected by serif rollback.
Original sites, save / published BEFORE January 18 look like this
ALL Sites Saved/Published AFTER January 18 look like this
Fallback and the like no longer work:
