on Debian/Ubuntu: "sudo journalctl -xe"
and watching around the time when app disappeared can tell something
I have found a hack which can cause Linux make app swap out its memory data, described in similar topic
https://forum.fopnu.com/support/90
systemd-run --scope -p MemoryHigh=512M --user nohup /usr/bin/fopnu &
Possibly makes sense if swap is on-disk or if it is compressed in RAM (
https://wiki.debian.org/ZRam#zram-tools ) while main memory is uncompressed.