PSNow and PS+ probably share the same product keys for certain titles. When you subbed to PSNow, it probably changed the service attribute from PS+ to PSNow. Letting your PSNow sub expire meant that you then lost access to the game, as your library JSON would point to that game being part of your PSNow sub, as opposed to PS+. I'd bet dollars to donuts that's what happened. I wouldn't expect a regular CS person to piece that together if they don't understand the DB structure.
I only assume this, because I have to dig through our DB regularly, and product keys are unique identifiers. And things like inventories shouldn't be deleted or modified by backend processes randomly. Based on your timeline, and what we know of how subscriptions work, games in your library are remembered even after you unsubscribe. So there must be a record of that game in your account still. What could easily prevent that game from being available now is if the service tag on it got overwritten. That would happen if they have the same PKEY/SKU in the system. That could be the case if both PS+ and PSNow access the same exact game data for a specific title.
To be sure, it would be poor design, but it might also be unforseen. If PSNow and PS+ don't use the same PKEY/SKU for most games, then this issue would only come up a small amount of times. Then shrink that number further to find the number of users that would report it. It might well be ticketed for correction, but could be a low priority. It might well be something else causing this entirely. However, based on how we know the system works, and what happened to the OP, this actually lines up. And fits in with how I imagine account data is handled. Each game in your account JSON would be an array with product keys, and service fields, among some of the data in the array. If the service field isn't designed to take more than one value, then it's going to be overwritten by the most recent service you downloaded that game with.
If you resubscribe to PSNow, and find that the game is once again available, then there you go.