A user brought this issue to my attention and at first I assumed it was some kind of user error. However, after extensive testing across different sites with different lists, it's clear that there is some kind of bug with the new "modern" date/time control in SharePoint Online.
What is happening is that when the time portion of a date/time column is set to PM (e.g.: 1:00 PM) AND either the month OR the day is a single digit value (e.g.: 01 for January or 01 for the first of the month), putting the field into edit mode will result in the PM being changed to AM. For example, 01/01/2022 12:00 PM with change to 01/01/2022 12:00 AM.
If both the month and day are two digit numbers (e.g.: 10/10/2021), the issue does not occur.
This occurs in Microsoft Edge, Google Chrome and Mozilla Firefox (all latest builds as of 11/19/2021) running on Windows 10 as well as the Microsoft Lists PWA (which, in reality, is using Edge under the hood).
It does not occur in any browser on Mac OS, so it's possible this is related to list sync (Project Nucleus).
UPDATE: I believe this is directly related to Project Nucleus. I went into the settings for a few of the lists that were displaying this issue and disabled syncing (Advanced settings > Set "Offline Client Availability" to "No") and the issue promptly disappeared.