Kmail does not load all mails in folder

Yesterday I discovered that Kmail has not been syncing my inbox for at least a month.
After deleting the account in Kmail and adding it again, things became even more weird.
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If you think having exactly 1200 emails in your inbox is unusual, you would be correct. If I check the account online, it has a bit over 2200.
The last mail Kmail displays is from March 2021, when it should be from 2 days ago.
If I delete old mail, Kmail will load more messages, until it gets to 1200 again.

Another mail account with the same email provider doesn’t have that problem, loading 1838 emails just fine after I deleted it and added it again.

I am using Kubuntu 24.04, Kmail 5.24.5 (23.08.5)


Wonder what exactly it is doing. Progress is updated once in 15 minutes and it still does not fetch more than 1200 mails from Inbox.