It's well established that with an appropriate warrant, LEO have always been able to come into your house without telling you and add hidden surveillance bugs to listen on your communications; they have always been allowed to physically modify or replace your phone (e.g. physical phone wiretaps a century ago); Electronic Communications Privacy Act reasserts that this applies also to electronic surveillance and digital communications; so (as a non-expert) I don't really see why that wouldn't apply to smartphone exploits as well. We do see exploits being applied to devices in LEO possession (e.g. https://www.theverge.com/2021/4/14/22383957/fbi-san-bernadin... for one random example) to recover evidence.
The main restriction is the constitutional limits of 4th amendment which requires specific warrants for each case - which is a significant practical obstacle, so the circumstances in which warrantless wiretapping is permitted (e.g. by PATRIOT act) is a contentious issue; however, it's not relevant if a proper warrant is obtained.