I had this weird issue recently: I had a simple Ubuntu setup with cryptesetup and swap, and the crypt swap device (say /dev/mapper/mycryptoswap1) keept disappearing.
Caution: fiddling with cryptsetup and disk devices is dangerous for data and OS. I personally made a full backup on a separate disk and then umplugged it to be sure it wouldn’t be involved in any mishap.
Turns out is a little worse that that: Linux is changing the drive devices assignations (eg /dev/sdb /dev/sdg) at every boot under my nose. I did not notice at first because I am using UUIDs, so everything looked fine. Unfortunately my raw partition did not have any UUID. Probably there’s a way to assign an UUID to a general partition, but I didn’t address this issue.
Having the disk changing its letter any time was the reason why the crypttab device wasn’t created a boot. I was lucky: would a partition with valuable data be present in the other disk, it would be overwritten with encrypted swap data.