Herndon I personally don't believe this to be a Android Pie issue as we've seen issues with Boxer sync breaking at random for the past 2 years now. I run into on Samsung devices more so then any other phone but I've had it happen first hand on both my Pixel XL and Pixel 3 XL. After the latest 5.5.0.3 update I've had a few users state that setting the app to manually pull email after a set period (example 15 mins) no longer seems to work correctly for them either. My director is one of the people having this issue and went so far as to unenroll and re-enroll his device this morning so he is going to give it a couple days and if it breaks again get me logs so I can get a ticket open.
Honestly management is starting to get frustrated with Boxer and has tasked me with exploring other options to see what we can come up with. When Boxer works its great but lately it seems like all the development focus has been on back end security features instead of UI Polish and reliability and since we still continue to have random sync issues they want a fall back plan in case things get worse.
Nate that sounds like a full time job 🙂