Lineage is just Android, however, some apps (Usually banking apps are the worst offender) throw a fit running on rooted/custom roms, though it's usually bypass able with varying amounts of effort. I would not expect a glucometer app to have issues, but I've seen apps throw that fit for less in the past.

Being a medical thing, I'd advise you to pickup a cheap used Android phone that's also on the supported list to test out your app first, or at least have an alternative means of monitoring it

I did some Google searching and it came up... inconclusive, though the Dexcom website has this blurb on the compatible phones section

"You can use this app on any OS that meets the minimum requirements, but Dexcom recommends not updating to a new OS before it's listed here."

And the only minimum requirement for Android is that it's v10. I also didn't see mention of root/or "unauthorized OSes" on that page and alot of the times they will put something like that if a companies app will scan for root/custom roms

So, you might be fine

Works on GrapheneOS

Out of curiosity, why do you think it wouldn’t work?