Intermittent failure to receive SMS and MMS messages
In my case, the issue with the two non-stock messaging apps was the battery settings. Android was throttling the apps when they weren't in active use to save on battery. For whatever reason, incoming text messages would be directed to the apps but the apps would never wake up and the message would fall into a black hole. The stock messaging app was set to never be throttled in the Android battery settings. Changing Signal to also never be throttled seems to have resolved the problem. This is a known issue for Signal: https://support.signal.org/hc/en-us/articles/360027913252-Troubleshoot-receiving-messages