Undo history stops working

Recently have found that Logic Pro X (latest version, 10.4.6) stops keeping track of the undo history after a while — sometimes after more than 100 undo steps, sometimes much less.

If I notice in time (e.g. by showing the Undo History window and watching it) I can save, then quit and restart to reset the history. But if I don't notice and need to undo ... I'm out of luck.

I can't see any reason for this, anyone know why LPX might be stopping its record of the undo history? I'm not running out of disk space, there's no other issue I can point to ... Any thoughts or suggestions from the Brains Trust?