Thanks for the quick reply!
One of those… as in there are others? I’m not aware of this issue being reported before. Do you use addons.cfg? Do other add-ons also disappear after a restart?
Searching through the community I found at least one other mention of having to reinstall NGRE on restart. I presume there are others. The only uncommented line in my addons.cfg is for the openhabcloud service. NGRE is the only add-on I have trouble with.
Maybe typical for you, but having to clear the cache on a system that has not changed sounds more like faulty hardware than software. A restart should be enough to clear up OOM errors.
Fair enough. I can be a belt-suspenders personality, and it seems like fairly common advice on these pages. But I’ll scale it back.
Is it not working consistently, or does it work sometimes? Was the ItemRegistry accessible at this time? After clearing the cache, do you restart OH at least twice? When this issue occurs, does restarting OH help?
The rule worked reliably before this one incident. No errors with the ItemRegistry after the restart. Other rules that operate on the item registry without using metadata were OK. I only restarted once after the start. Rather than stop/clean/start/restart, I can be a restart/restart person. I can’t see how to recreate this issue, but if it happens again I’ll see if that is sufficient.
I suggest writing a script to populate/modify the metadata for your Items.
Sensible suggestion. Put it on my list of things to do. I’m an amateur – this rule is only the second piece of JavaScript I’ve written (better to say cribbed from these pages). It’s a bit of an investment, but step by step, right? Maybe I can get my daughter to write one for me 
Really appreciate you and the other pros here who are tirelessly helpful to us dabblers.