I think this happened sometime in the past week, not sure exactly when, but all of my account bank pages have reset to their default name (Page 1, Page 2, etc.).
Fortunately, everything else about my Wynntils data seems to be intact as far as I can tell, such as favorites and waypoints, it's only the bank pages that got thanos snapped.
Unfortunately I have no idea how to repro this since I'm not sure when it happened. Kinda hope it doesn't happen again, but I guess I'll at least be anticipating this time.

I stand corrected: looks like my item record also got wiped at some point. Although I'm not sure if it happened around the same time or not.
Is there an input/output limit on the JSON lib you are using by any chance? Just to rule out the most obvious possibility

Decided to take a look at the mod's automatic backups folder (Thanks for that feature by the way, because I totally forgot to make any backups of my own) to try to get a timeline of when the data loss may have occurred.

• 2024-08-14 through 2024-10-06: file size of .data.json snapshots steadily increase, up to a size of just under 7 MB on Oct 6
• 2024-10-06 through 2024-10-08: The file size of .data.json snapshots jumps from the aforementioned 7 MB down to a mere 3 kB, suggesting a total loss, a reset, in other words of the on-disk file.
• 2024-10-08 onward: snapshots steadily increase as before, suggesting no further data loss.
  I'm not quite sure what happened around this time to cause the entire file to be reset... Might have to think on that and do a little digging through relevant logs.

Also, earlier in the bug report I mentioned that some of my data was not lost... I realized you actually store stuff like waypoints and favorite items/ingredients in the .conf.json file, not the .data.json file (Why do you do this?), which probably spared them, by dumb luck, from whatever ravaged the .data.json.

Well, whatever went wrong exactly, restoring the backup (I renamed the active version to append "statemachinebroke") did not seem to cause any immediate issues. Funny enough, when I loaded into the server, it read the data file and "oh, you haven't played since Wynntils v.X.X.X, have some release notes friend!", so once again, thank you for having a sane and workable autobackup feature.
Still no idea why it lost the data file mind you, so this is still an open issue. I'm just grateful for the contingency.