I tried @Martijn S's mod and had my test account logged into my game while I was doing some editing. I noticed later that when the screen saver on the test computer had kicked in, the online status changed (I don't know if the avatar also disappeared, as I didn't think to look at it until after I'd woken the test computer up; my focus had been on updating handouts, not looking for players). So the root cause here could simply be people switching to other apps/tabs that cause the browser to "pause" the tab containing roll20. Both Firefox and Edge at a minimum have that, and at least on my test copy of Firefox it was enabled (about:config, browser.tabs.unloadOnLowMemory) and I don't recall ever tweaking the Firefox config (it's on a username dedicated to roll20 player-view testing). It's possible some aspect of this changed recently to make it more aggressive (MS's docs say Edge won't pause a tab until 2 hours of inactivity, for example, and my player was only inactive for ~10 min at most). Most of my players use Edge, although a few (like me) are Mac users and use Firefox. I'm not sure which Saturday's player was using.