In the Marley Arc many have speculated that Eren and Zeke have been working together and while he does have moments of suspicion like the time he changed a topic of conversation between himself and Colt or him walking away from the festival, Chapter 102 kind of makes it less likely but does not yet completely disprove the notion that Zeke might be on the same side as Eren. While Pieck's reaction to Zeke's entrance could indicate that she is suspicious of him, I think it's just a fairly standard reaction considering her personality. I think it might indicate that she is wondering where Zeke was and what he was doing while she and Porco were trapped, but not necessarily, I think you're working with the Devils of Paradis Island. We know that Zeke is publicly on the side of Marley/Liberio but in actuality, I myself believe that he is actually on his own side and I believe that he has been loyal to himself since the time he turned his parents and the Eldian Restorationists to the Public Security Authorities. We have to remember that Zeke is pretty much the only Eldian to have both Eldian (Restorationist) and Marleyean propaganda deeply ingrained into him and because of that, he is more immune to the brainwashing than most other Eldians living in the Liberio Internment Zone though he will say the occasional devils of Paradis Island thing from time to time. And if there's anyone Zeke hates more than the Walldians, it would be his father, Grisha Yeager, and he would be quite justified considering that Grisha forced his beliefs onto Zeke and never really gave him the chance to be a normal child.