Missing emails

To all you geniuses out there who clearly know way more than I do (which, let's be real, is basically everyone) 👋

So, we’ve been using this one main, general-purpose household email account for about a decade now—you know, the one for everything from banking and shopping to business deals, negotiations, and all those random confirmations... it’s an unknown AOL account.
On my computer at home, I usually run Mozilla Thunderbird, though I occasionally hop on the webmail if I have to, but honestly, I just use Thunderbird on my laptop too whenever we aren't home.

For ten years, everything was totally fine and easy; I’d just clear out the junk, the notifications, the expired offers, you know what I mean... and keep the important stuff: account confirmations, receipts, contracts...

Then, back in July, out of our roughly 2,000 emails, only about a hundred remained from July 1st onwards—everything else just vanished into thin air.
It’s not in the trash, it’s not in some other folder, it’s not even showing up on the webmail.
We didn't touch any settings, we didn't mess with anything...

Anyway, time passed, and I figured, whatever, it's probably not that big of a deal... but then it happened again today. Now there are only four emails left, and they're only ten days old, even though there used to be about 200.
They aren't four consecutive ones, and they aren't even from the same sender; I can't find any pattern to explain why these specific four stayed. Even the ones that were skipped during that first "wipeout" are gone too. So yeah, just four emails left.

The folks over at AOL support have no clue what's going on; they just tell me to change my password, suggesting maybe some virus crawled in and messed things up.
OK, so what do you guys think?

What kind of Scooby-Doo mystery is this, where mail that was sitting right there in my inbox this morning—without us touching it—is suddenly just gone?
I can still send and receive mail normally, and things stay in the inbox for a bit, but how am I supposed to trust anything when it might just disappear again? And where on earth did all our important old emails go?

Emily Ortiz27 said:To all the experts out there—which, let’s face it, is everyone else here 👋

For about a decade, we've used an iskonov email account as our main hub for everything: banking, shopping, business negotiations, invoices, confirmations... you name it.
At home, I use Mozilla Thunderbird, though I occasionally jump on the webmail. Not often, though; I have Thunderbird set up on my laptop too for when we're away from the house.

Ten years of smooth sailing. I’d clear out the junk, the notifications, the expired offers, the absolute nonsense... leaving only the essentials: account confirmations, payment receipts, contracts...

Then, this July, everything went sideways. Out of roughly 2,000 emails, only about 100 remained from July 1st onwards. Everything else just vanished.
It's not in the trash. It's not in some hidden folder. It isn't even showing up on the webmail interface.
We didn't touch any settings. We didn't mess with anything...

Time passed. I thought, fine, maybe it's not that big of a deal... but then today happened. Only four emails were left—all less than ten days old—when there should have been around 200.
They aren't consecutive, nor are they from the same sender. There's no pattern to explain why those specific four survived. The ones that escaped the first "purge" are gone now, too. Just four measly emails left.

The support staff at the ISP has no clue. They suggested changing the password, implying some virus might have crawled in.
Fine. But what do you guys think?

What kind of Scooby-Doo mystery is this? How can mail that was sitting in the inbox this morning—without us touching it—just be gone?
Sending works, receiving works, and things stay in the inbox for a moment, but how am I supposed to trust anything if important emails just evaporate into thin air?

Are you using that account on multiple devices where one uses IMAP and the other uses POP3? POP3 might be pulling them off the server after a few days, which would then sync to the IMAP account.

Did the sent messages disappear as well?