Board Thread:Manga/@comment-38849221-20190321163015/@comment-30281369-20191006225311

Personally I think it had both to do with trust issues as well as lack of information regarding the founding titan's powers and how impacting it's utilization would be. Most of Zeke's (as well as Marley itself) information on an outsider using the power came from Reiner and possibly Bertolt, and I'm sure one of the key points that they mentioned was that they were able sense Eren using it.

As such from Zeke's perspective, if they were able to sense it then, then they would be able to sense it again, as well as potentially recognizing the sensation as Eren using the founding titan's powers once more. In short, them coming into contact would more than likely had alerted their marley and their titan's, which was an unnecessary risk. After all, we still don't even know if Eren would be able to control titan shifters.