I'm probably using an old name for the setting. I've enabled it under Settings > Display > Lock Screen > Always show time and info. After enabling this option, I still had missed calls or delayed notifications. As soon as I touch the screen to "wake" the phone, even though the screen is already on, the incoming call notification pops up immediately.
If I leave the phone plugged in to power, I don't seem to have any problems with missed calls or delayed ringing. In this case it doesn't seem to matter if I use a screensaver to keep the screen powered on or if I let the screen completely power off.
A recent post on the JMP blog mentioned that a new version was released or soon to be released. JMP support didn't say anything about possible fixes in the new version though. They specifically noted a change in GrapheneOS about five or six months ago that seemed to be the start of the problem. I'll try out the new version though as soon as I can get it from F-Droid.