The TL;DR on Apple's macOS Mojave 10.14.4 Gmail/G-Suite Issue: It's Fixed
Same—issue still occurs for me. I haven't had any success by clicking the offline symbol for all accounts, but if I click them individually, it forces them online one-by-one. However, I have to do this constantly throughout the day.
This is a very different problem than the can't-authenticate-at-all issue that was solved a few weeks ago.
I, too, see my accounts offline from time to time, and those times are far more frequent since the 10.14.4 upgrade. But a click or relaunch of Mail fixes it in all cases. Not ideal, but mostly just an annoyance.