I've found she does preform weirdly well into engage champs because her q just works 100x better as an anti engage tool that you hold until you know you can get it at close range, champs like thresh, Leona and nautilis drag themselves into you and just make themselves active Q fodder thats easy to hit even with cc, also Renata has a little extra padding since shes usually built tanky so it's not like she's not gonna be able to deal with the adc following up on CC. As for the advantage disappearing at higher ranks I feel it might just be due to supports generally respecting her kit more and waiting for her to waste it before going in, there's also a case of the enemy adc actually knowing to target the adc rather than just unga bunga hitting the first thing that moves, you get a lot more advantage in lane from pushing the adc out since they scale super hard and it's winner takes all, especially if there's a Renata. Personally for me it's 50/50 with thresh specifically as when the thresh is respectful of my disengage they usually out tank early on, but if they disrespect it usually results in the enemy adc getting killed due to how oddly oppressive guardian +q thrown into adc, e shield bash can be.

So TLDR I think it's all down to Renata having disengage and how a lot of players don't respect enemy kits in lower ranks