Sounds like a good idea! Do you reboot the server or just the container?
I’ll call this the “solution”, if only for tidiness. 
Anyway, I had it happen again on another 2gb+3GB server. Then I rebooted web_only and tried again and it worked just fine. I think I’ll add to my tooling to reboot web_only, perhaps if memory is some definition of “low”.
I didn’t do anything to disable it.
I do that on machines that I set up . . . and it looks like this one has that turned on already.
Thanks to everyone for your ideas!