Since the change in the city border display (I believe it went from double lined to single line?), I have a really hard time seeing the border when you have a busy heatmap overlaying it. I don’t know if a colour change might help? At least for me, the blue and purple (I think these are the colours) just blend together. I do have some red-green deficiencies though, which might be related to the issue.
I have the same problem. There should be a setting for city border colour. I would definitely choose red or black. I really don’t distinguish between blue and purple.