Bring up VirtualMin on port 10000 in a browser. Log in. Choose your domain in the VirtualMin pulldown menu.
Expand the Server Configuration item. Then choose PHP Options.
On that panel, when FPM is chosen (if you have a choice), there is the item
PHP FCGId sub-processes
adjust that to be 5 or more (or whatever you see for pm.max_spare_servers) – this value is used for pm.max_children for this domain. Then start php-fpm for 8.1 back up again. You will have to do this by hand for each new domain.
I have no idea how to fix the root cause (well, if you are very brave, you can edit the VirtualMin PL scripts/files 
– that is, how to make VirtualMin handle this issue). But you can adjust your Server Template for all future domains … in VirtualMin, expand the lower menu section System Settings. Then choose Server Templates.
Once there, choose Default Settings, or make a new custom one for yourself. Once you are viewing a Server Template, use the pulldown to choose PHP Options. Once on that panel, adjust
Default FCGId PHP sub-processes
to be 5 or more (or whatever you see for pm.max_spare_servers) – this value is used for pm.max_children for all new domains.