Hello Mark, thanks for reporting this.  I was able to replicate it so I'll make a bug report on it so it gets fixed in a future build.

This was fixed in Build 6919 (Dec 11, 2018):

There still seems to be an issue with this in Build 7068. Folder Auto-Clean options are not working when you have them set to clean up folders by size. I tried setting various different rules on multiple folders. The test folder I used had 19.6 MB of email in it. Various rules included - 

When size is over 5 MB, reduce to 1 MB

When size is over 1 MB, reduce to 0 MB

When size is over 0 MB, reduce to 0 MB

None of these will trigger. This is happening on multiple domains/email accounts. "Override auto clean settings" is enabled. Tested various rules by Age, and those seem to work just fine.