Same issue here. Legacy version on Mac is now apparently as unstable as the iOS mess.
When my Mac upgraded to the new release, multiple fails were immediately obvious --> I rolled back to Legacy. Looks pretty bad but has been functioning reliably on the laptop ... until today. I worked all afternoon in a note (previously saved and synced multiples times in the morning). Left my laptop open at 4PM. Came back at 7PM, caught a glimpse of all my text before it erased itself in front of my eyes. A couple hours of work gone.
Checked the server: nope. And I didn't touch the note on my phone in the gap, so there's no way it's a sync conflict. No explanation. Evernote just....deleted it. Rolled back to a version from 11AM...while synching? Which it didn't do for several hours despite the laptop being on and the program open??? FFS. Why use a data app that erases data??
Insult to injury: the "versioning" that used to create versions of notes multiple times/day and used to be a partial fallback in the old days when this behavior was more common? At some point last year, EN cut back from saving maybe 12 versions/day to 3-4 across an entire day ---> completely useless feature in situations like this. (Case in point: the versioning feature saved four versions of today's lost note, three of them in the morning when I made minimal changes. It saved ZERO versions between noon and 7PM when I quadrupled the size of the note. Well done, Evernote!)
Getting so close to done with this trash behavior. Good for Evernote for finally realizing their platforming was unsustainable, but who goes about a massive upgrade by undermining the entire raison d'etre for the entire company - the CORE FUNCTIONS of saving and syncing data? I guess the idea is they'll just build an entirely new user base on the new platform, having alienated everyone who's stuck with them for 10+ years? Helluva marketing plan to go with a half-assed rollout....