I have that problem with one of my local highway patrol channels. It has a constant interference/background noise on that channel. It won't stop the radio from scanning, but once it stops on a voice transmission on that channel, the background noise is enough to keep the squelch open and stop it from resuming scanning after the voice transmission ends. My solution was to program the PL for that channel. My radio will then resume scanning even if there is another signal (such as noise or interference) on the channel as long as the programmed PL is no longer being transmitted.
Sometimes this problem can be solved with antenna selection and/or placement to minimize or eliminate the noise, but that's a hit-or-miss proposition (not to mention a lot of effort).
It sounds like your radio is sensitive enough to pick up other users on the same channel. This is exactly the situation that the use of PL was designed to deal with.