On my iPhone + iPad + iPod (yes, still have one), there is a folder in the Mac/iCloud for InboxSpamSieve identical to what is on my Mac desktop Outlook/O365. The difference with the iOS versions is that they are all ‘hanging on’ to some ‘good’ emails going back as far as 9 months. It is not every email, just several (32) that have passed through the system. There aren’t any issues with any of them. From a desktop standpoint, all of these emails that remain on the iOS devices have been deleted, read, or flagged for later. Since this folder on the iOS devices doesn’t seem to cause any problems, I am more curious as to what is happening. Maybe the best thing to do is just ignore it because the SpamSieve magic happens regardless?
It’s probably fine to ignore it, but it sounds like Outlook on your Mac is either set to hide those messages or perhaps moved/deleted them locally without syncing that change to the server. Do you also see those same messages in other mailboxes on iOS? If so, you could delete the InboxSpamSieve versions.