Are iPhone iOS 18.1 updates still causing issues?

NO updates in the past 13 years have caused problems, so no, 18.1 is not causing problems and hasn't. But the current version is 18.4.1 as of April 15, 2025, and you should update to it, because it fixed over 50 vulnerabilities since 18.1.

Some users have problems after an update, but the ONLY time an update caused a widespread problem was in 2012. And none of the observed occasional problems (except for the 2012 one) were actually caused by the version; they either were a problem installing the update, or they were related to an app that was incompatible with the latest version.