This makes dev-vda5.device go down and back up, and thus looks like it got hotplugged, and then the fstab entry for /dev/vda5 gets triggered which auto-enables the swap partition again.
This remove/add event can happen if partman removes the existing partition and recreates it, which is fairly plausible.
I'm able to replicate this behaviour with the following in QEMU:
swapoff /dev/vda5 match=vda5 match=vda5
udevadm trigger --action=remove --sysname-
udevadm trigger --action=add --sysname-
This makes dev-vda5.device go down and back up, and thus looks like it got hotplugged, and then the fstab entry for /dev/vda5 gets triggered which auto-enables the swap partition again.
This remove/add event can happen if partman removes the existing partition and recreates it, which is fairly plausible.