Sadly, it's pretty much up to your carrier, as I suspect it's still a matter of your local cell tower assigning your 12 Pro Max to a frequency the tower does not handle well due to configuration, local conditions or whatever. Because the 12 Pro Max can handle that channel, it's being assigned to that channel and your experience is the result.
For example here are the FDD LTE frequency bands supported by both devices:
- XS Max: FDD LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 14, 17, 18, 19, 20, 25, 26, 29, 30, 32, 66, 71)
- 12 Pro Max: FDD-LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 14, 17, 18, 19, 20, 25, 26, 28, 29, 30, 32, 66, 71)
If the cell tower base station controller sees the 12 Pro Max can use band 28, which will be lightly congested as not many other phones use it, but there are issues with those frequencies due to tower configuration, foliage disturbing propagation, etc., you'll see your XS Max have no issues but your 12 Pro Max will drop the signal. When it rejoins the network, the controller will again see "Oh, a device that can use Band 28" and… rinse and repeat.
It's really cellular carriers trying to wash their hands of the issue and leaving customers stranded if you will as a result.
To "fix" this your carrier would need to send a tech team to the site, run tests on that band, notice that certain bands have issues and either fix the issue at the tower if possible, or program the controller not to assign devices to that band - all of which require an expensive crew to go out to the site and spend time there. This is also why when that does happen, suddenly the phone starts working with or without a carrier settings update but without an iOS update.