Ubuntu 18.04 Swap turns itself off

I allocate a swapfile, and turn it on:

sudo swapon /swapfile

I can see it here:

$ free -h
              total        used        free      shared  buff/cache   available
Mem:            62G        6.0G        7.1G        6.2G         49G         50G
Swap:          199G        256K        199G

I can see it here:

$ sudo swapon --show
NAME      TYPE SIZE USED PRIO
/swapfile file 200G   0B   -2

I can see it in dmesg:

[373676.694883] Adding 209715196k swap on /swapfile.  Priority:-2 extents:808 across:258359292k SSFS

Yet after some time, it no longer shows…

$ free -g
              total        used        free      shared  buff/cache   available
Mem:             62           7           0          12          54          42
Swap:             0           0           0

And this

$ sudo swapon --show

Returns nothing.

dmesg gives no error.

It’s like it turned itself off.