don’t know the context, but I just had an objective game on Terminal in which as defenders an ally of mine planted the secondary objective and I would still get the plant notification, like this:images.akamai.steamusercontent.com/ugc/383162071970909637/FB79F9DDA4071CE2D985F87CC78D0261C69C7000/
I could iniciate the planting animation for a fraction of a second, repeatedly but couldn’t effectively plant a second bomb.
Same for me. On defending team at terminal. Happened on objective and stopwatch. Pretty sure it happend on underground too (first two generators but as attacking team), but ill double check.
Edit: After few underground games, this bug was NOT there.
Yesterday got same bug on bridge map. I was on attacking side playing proxy, i have planted C4 on generator for barricade. There were both icons displayed, one for defending C4, other was for destroyable objective, like in the picture Lumi posted. I wanted to take ss of planting, but that half a second when is to little time to get a good ss 
(+all the evil ppl shooting at me)
On short… same bug happend, but on diffrent map (bridge) and this time i planted C4.