NetHack has recently undergone internal changes for map representation
and rendering, among them some changes in color handling for objects.
In the process, the bug you've discovered crept in. Your post prompted a
fix which is already in place in the sources. It should be gone in the copy
you're running after the next time hardfought updates the 3.7-dev version.
In the mean time, you might be able to work around the issue by using
'O' to set 'symset' so that nethack will update the map stuff. I'm not sure
whether you will need to change the value to something else and then
change it back, so just select the current value. (I haven't tested this
workaround so it might not actually work. Conceivably save and restore
could fix things up too since only new games are impacted (an oversight
after object description shuffling), but again, I'm just guessing here.)