I have noticed this "bug" in the terrain map page of the app recently. I have updated to version 19.23 and this bug may have already been present in earlier versions. This video sums it up:
I am using a Samsung Note 4 and previously have not experienced this problem.
ADD: I think there is a problem with the algorithm where it calculates AGL using the glideslope/climb angle of the aircraft as reference. So it seems that while descending, the far areas become purple because those areas are above the glideslope. Is this a BUG or a FEATURE?