If you open the map and do some stuff (scroll around, change scale, mouse over the map), the pollution may disappear or reappear on some areas.
It is kinda weird to reproduce, but seems like it disappears on scrolling below and reappears on scrolling above your current position.
[0.13.4] Strange pollution display behavior on the map
Re: [0.13.4] Strange pollution display behavior on the map
Duplicate of viewtopic.php?f=182&t=27970, apparently resolved for the next release.