[bitfolk] Oddity with one UBUNTU 22.04 VPS

I am responsible for several VPSes, here and elsewhere. Five of them are running Ubuntu 22.04, three Debians. The script I use to update them checks, at the end, for the existence of /var/run/reboot-required If it finds it, it offers to reboot the VPS. It does this happily on all but one VPS, one of the Ubuntu ones here. The Ubuntu version of apt-get on all of the Ubuntu ones recognises that a reboot is required after a kernel update etc and will popup a message saying so, but it looks like only on this single machine, that file doesn't exist afterwards. I have no idea why not. Anyone got any ideas? Ian