Thunderbird no longer syncing Office 365 "Sent Items" folder

Late July 2025, I received a Thunderbird update which caused the UI to change and came with a couple of regressions (e.g. I can't tell from the quick filter buttons whether or not they are active anymore). Ever since that update, Thunderbird is no longer syncing my Office 365 "Sent Items" folder. I have configured my Office 365 account as follows: - When I send an e-mail on the account, I configured Thunderbird to *not* place a copy in the account's "Sent Items" folder, because the SMTP server already does it. I can check in the Outlook Web Application (OWA) that this is actually still happening. - This means that sent e-mails do not immediately show up in my unified inbox within Thunderbird (which includes the "Sent Items" folder), but normally they do after I open the "Sent Items" folder, triggering a synchronization. Since the update, the "Sent Items" folder in Thunderbird shows as empty. No old sent messages and no new ones are visible. In OWA, all of them are still there. In fact, the same seems to be true for other folders (except inbox and trash) on the Office 365 account. Of course, what I want(ed) to know is: how do I fix this? I am on Ubuntu 22.04 and currently Thunderbird 128.12 (but further updates may have happened since the regression occurred.) **Baffling addition:** While typing this question and looking through all of these folders on both OWA and Thunderbird, suddenly they synced. All of them. I changed one thing, but only for the "Sent Items" folder (so not for the other folders that were not, but are now, syncing): I right-clicked it in Thunderbird, chose "Properties", and checked "When getting new messages for this account, always check this folder", and then restarted Thunderbird. For now, I conclude that the interaction between Thunderbird and Office 365 is non-deterministic.