Does this happen only on the map you made? If so its probably corrupt and theres not much you can do... Otherwise just tinker with the graphics settings - I think I remember having this issue about 5 years ago and it was one of the settings which disagreed with my gpu

thank you for your response. Adjusting my graphics settings sorted it out.

@casaha if you want to test your map, the fastest way is to use the world builder from Documents\my games\Company of Heroes, then when you export you can simply open Company of Heroes - Legacy Edition and your map will be in the list.
In your situation, either transfer the files to from their respective folder in Company of Heroes Relaunch to Company of Heroes
or
run company of heroes (new steam version) with -dev target line
First option will be faster and easier in the long run

The single player thing might be the problem. I don't know tho because I've never attempted to make a single player mission.
In any case, copy the folder Documents\my games\Company of Heroes Relaunch\ww2
and paste (and overwrite) to Documents\my games\Company of Heroes
then open coh legacy edition and see if its there.
Otherwise if the problem is the single player thing you might try using the stamp tool in world builder to copy-paste your map to a multiplayer format. (You'll have to do some googling there to find out how to do that Im afraid)