I have this issue, too, and for a long time. Actually since I first installed the eSIM, when GrapheneOS implemented it.
I'm not pretty sure when exactly the toggle goes off, but after a simple reboot and installing a new GrapheneOS-update could be right.
I know, it just a simple toggle to turn on, quite easy, but more often than it should, I forget to turn it on again.
In that case I'm not available by phone anymore with that number from the eSIM (but the other number with the inserted SIM still works) and I had to turn it on manually. The problem: That's my business number and I receive regularly messages, that the number doesn't work. (and that's the hint for me, that I forget it again)
So please fix that, if it's possible. Thank you!