If your DE/Launcher uses systemd scopes properly you might be able to see something in the journal. As an example somewhere in my logs I can see this:

Jan 17 17:52:50 sky systemd[2171]: app-niri-steam-40213.scope: Failed with result 'oom-kill'.
Jan 17 17:52:50 sky systemd[2171]: app-niri-steam-40213.scope: Consumed 6h 32min 39.773s CPU time, 9.4G memory peak, 6.2G memory swap peak.

That's pretty clearly severe thrashing and an eventual OOM event caused by a game. If you're not familiar, the command journalctl -e -b -1 gives you the last log lines from the last boot. Use d and u to navigate the pager and q to quit. This will only work if the launcher you are using sets up transient systemd scopes and doesn't just fork-exec into the application (Fuzzel does the wrong thing by default, as do many others).

I've also seen large Steam downloads causing such issues, so capping your download speed might help. As could enabling ZRAM.

Edit: Also, this is most likely completely unrelated but do note that Neon is basically abandoned. You should very much consider switching to a maintained distribution, whether that's another Ubuntu spin or Fedora or something else entirely.