It may help to share your settings for how deletions are to be done for that account. It may also help to share if there are any similar settings that are set online for the account. (I mention that because Gmail is an email provider with such settings and there may be others.)

My IMAP account is indeed a gmail account.

I have attached images of the gmail and thunderbird settings that looked like they might be relevant to deletion.

I'd be surprised if different choices for these settings would (intentionally) be responsible for the difference in behavior between the ordinary inbox and the unified inbox.

I'm not sure of what you want. What you describe reflects the settings you posted. If you want the messages to delete as they do for the POP account, then set the account to move to trash.

My question is: Why does a deletion from the single-account inbox work differently than a deletion from the unified inbox? I don't see anything in the settings to suggest that it should.

(I have some other confusion about IMAP and/or IMAP-with-gmail, but that's not what prompted this posting.)

You say you "added" to account to the inbox, I suggest you delete all the the smart folders from your profile folder and allow it to regenerate automatically You should never have to manually add an account to unified folders. That you did says there was an issue way back then.

But you have auto expunge turned off, so immediate deletions will not occur until Thunderbird notifies the server of the action, generally that is when the application is closed.

You have mark as deleted turned on.... that is the strikethrough. I don't think smart folders even understand the concept correctly and apparently neither do you, so perhaps allowing deleted mail be deleted will remove the apparent anomaly that comes from not knowing what mark as deleted means.

You say you "added" to account to the inbox, I suggest you delete all the the smart folders from your profile folder and allow it to regenerate automatically You should never have to manually add an account to unified folders. That you did says there was an issue way back then.

I didn't add the account to the inbox, I added it to Thunderbird. I apologize that this was apparently not clear from my original posting.

I have deleted smart folders multiple times during the time Thunderbird has known about this account.

But you have auto expunge turned off, so immediate deletions will not occur until Thunderbird notifies the server of the action, generally that is when the application is closed.

Are you saying that the server will not see deletions until I close Thunderbird? Because that's not what I observe (i.e., if I look at the inbox from within gmail itself, I do see the deletion/archiving having happened before I close Thunderbird).

I dont think smart folders even understand the concept correctly

If so, then that explains what I'm seeing, and seems to be a bug.

and apparently neither do you

If I misunderstand this concept, then I'd appreciate it if you could explain it or point me to an explanation elsewhere.