Curious, Charlie. I have Mojave, APFS and I use Time Machine Editor to do two backups a day, noon and midnight. In between TME turns TM off, and the Back Up Automatically is unchecked. As of right now, it's been 9 hours since the last back and executing "tmutil listlocalbackups /" yields two, one today at 15 minutes after midnight and one 15 minutes after noon. So, basically, with the box unchecked, no snapshots, but when TME turns it on it does one while the backup is in progress.
So, basically, with Mojave and APFS, if Back Up Automatically is not checked, no snapshots. Only when TM is invoked do you get one snapshot, which I can live with.