This seems to be a semi-common issue with the Verizon G1100 router I have. Like, not that it’s broken, but more that it’s a terrible piece of hardware 
I’ve disabled UPnP, and added the port forwarding rule manually to see if this solves the issue. I did see another forum discussion where others were having a similar issue, and adding the rule manually seemed to be the solution. (ARC is simply unusable for me - #73 by John_Cibula - Thanks for the link, Rugby!).
When I had UPnP enabled, Roon would automatically create another port forwarding rule in the router, even after I deleted theirs from the system, in an attempt to only have my manual rule. So now, with UPnP disabled, only my manual port forwarding rule is in the router. I’m curious to see if this helps. Fingers crossed.
If this does work, I’m hoping I can re-enable UPnP down the road, and not experience any repercussions.