As Box suggested, there is an issue with SketchUp 2017 when running on M1 Macs to an external monitor. Here is a long discussion about it, and at the bottom someone found a utility that solved the issue:
We are not going to release an update to 2017 to work around the issue, and so far it’s looking like Apple won’t fix it on their side. It may be worth trying the utility as a solution.