One of the most important ones is this one: https://joint-research-centre.ec.europa.eu/jrc-news-and-updates/stemming-tide-beach-litter-2018-09-25_en

This was the basis for the decision to mandate the caps be attached to the bottles, since they found a lot of caps and less bottles. This would indicate a lot of caps get separated from their bottles, which this change should mostly fix.

The other was an interview with this guy: https://zwerfinator.nl/ I can't find the interview, no idea why, but he hasn't published the results yet (no idea why, it was scheduled to be published I think, but somehow got delayed). They were saying the new pfands really helped and the number of bottle caps have gone down. But this is highly depended on the location. For example a lot of research is focused on beaches, where there is obviously often a large delay between the deposition and the collection. In city centers this time is often much shorter, so the impact of changes are seen faster.

So sorry to disappoint, it's too soon for a peer reviewed study diving into this. Also with all the other changes the EU has mandated on litter and single use plastics, it would be hard to quantify which implementation has what effect.

But my thinking was: There is for me good data showing this is an issue (which I was doubting) and the solution seems solid enough. Other changes like pfands on small bottles and cans have made a big impact (research is available for this, for example https://open.overheid.nl/documenten/ronl-8b11214f23388b3395402609d76286475b4f2908/pdf). And the people doing the research say they've seen results that it works. So that was enough to convince me.

If somehow the caps being attached doesn't lead to less caps in litter, that would be a very interesting result. Without a doubt the EU would change the regulation to fix it, depending on why they think the change didn't work. But this would not lead to the caps being like they were in the past.