God that playback queue has been so awfully buggy for so long. Rant incoming.
I actually discussed it with a friend a few years back while debating the declining quality of Google's engineering.
The craziest thing to me - it appears to be using some kind of eventual consistency, so additions/reorders/deletions have to go through some complex process server-side that takes several seconds to update in the UI (and often the order is wrong, or silently fails to add videos). And yet, the whole queue disappears without a trace if the YouTube app gets unloaded by iOS, and is unavailable on other devices, so it could have just been stored locally all along.
I thought that perhaps they were storing it server-side because eventually it would allow the queue to be restored or transferred, but it has been about 3 years now and I don't think that day is ever coming. I just lost a whole queue of several vids this morning (on the plus side it was a good incentive to get off YT, so I'll give it that).
My guess is it's using eventual consistency or some complex multi-microservice chain of RPCs because that's just what you have to do at Google. I'm sure there are engineers who want to fix the feature or go back and complete the rushed launch but likely can't convince the decision makers that it's necessary.
So we get a subpar experience from one of the largest companies in the world with thousands of the best engineers, while Google keeps getting their $16/month because there are no alternatives