It is being added to your queue of rewards. They've removed the option for you to see what it is first. They now blindly get added to your queue and they are retrieved First In First Out. So any new ones added, go to the back of the queue.
Basically, what this means is that encounter stacking, beyond 2 weeks, is dead. The reason is that you can only claim 1 research breakthrough at a time. What you used to be able to do was immediately claim the research breakthrough, catch the pokemon, and it would not enter your queue, allowing you to carry on queuing rewards, while also claiming your research breakthrough stamps. Now, if you want to queue beyond 2 weeks, you have to give up your research breakthrough reward.
I like the change personally, as it should have been from the start, but annoying that starpiece, mass catch sessions are now heavily nerfed.