Just a follow-up, since there's been no reply. I've concluded that it's related to or at least severely exacerbated by this issue in 24.11 with the dashboard impacting the system load: https://redmine.pfsense.org/issues/15969
It's kind of like the observer effect--it seems most prone to happening when I'm investigating it happening, or more particularly, when I've accidentally left a tab running the dashboard open. Earlier this week I logged in to get some info on a DHCP lease, forgot to log out and went on my merry way, came back a couple hours later to find that most of the OpenVPN connections had gone down again, plus there were a bunch of entries in system.log relating to php-fpm and connections being refused for loading the dashboard widgets. I restarted php-fpm and the gui from the console menu, and the VPN connections all came back online within a short period of time.
I'll be glad when 25.03 comes out so this problem is fixed!