This is really strange - even with persistent journalctl logging the logs before this scenario will be vanished. This also seems to not depend on the battery percentage since my Tablet also shut down with full battery.
[root@Jolla nemo]# journalctl --no-pager | grep "shut" Mar 17 23:58:56 Jolla kernel: dollar_cove_pwrsrc dollar_cove_pwrsrc: Last shutdown caused by SOC initiated cold off Mar 18 00:11:52 Jolla kernel: dollar_cove_pwrsrc dollar_cove_pwrsrc: Last shutdown caused by SOC initiated cold off