Well, I won't get into what does or doesn't constitute a bug, but will try to better explain what I've tried and why it isn't providing an expected result.
I have a folder of .html files that are not being imported into Evernote using the Tools>Import Folder. I am only wanting to do this as a one-time workflow (i.e. once I've gotten these imported I will remove the connection to that folder). I have the Import Folders options set to import to a notebook called Inbox and to retain files in the folder.
I have confirmed that the files are valid using my browser. I have confirmed that the items are not being imported into Evernote - no new items have been created in the target Inbox notebook and I have checked the desktop app, the web app, and my synced Android app. I have also confirmed that the total number of items in Evernote has not changed, so I don't think they're just getting hidden somewhere.
The filenames of the .html files are unique and do not repeat.
I have not yet tried changing the extension (sorry for the mixup, Android likes to autocorrect html to HTML - the files are currently .html), but I will try that in a little bit.
Again, for clarity, this is what I'm trying to do: https://help.evernote.com/hc/en-us/articles/208314308-How-to-import-from-other-note-apps-into-Evernote