In 1.1, the click-drag placement starting from a pole with inbuilt lighting seems to be detecting the light as the starting point rather than the pole, resulting in the first placement next to the starting pole rather than at max distance. Discussion here: https://forums.factorio.com/viewtopic.php?f=23&t=92371

Any ideas?

Hmm it does sound like a core game oversight. Maybe reproduceing it in cleaner ways so it can be raised as an oversight, rather than a bug is a better route.
As I suspect putting any entity on the power pole does it, i.em just script create a belt on the power pole.

Or maybe there's some additional flags I can add so it doesn't get triggered.

I'm hopeful this may get fixed when they resolve some of the other power pole placement bugs floating around. So will leave this for a while and check back later. I am hesitant to look at doing anything major to mitigate a base game mechanic issue.

Looks like 1.1 is getting toward a stable state.