All allegedly means in that context is that someone other than the person you're talking to is the originator of whatever message is being conveyed. When I say that something "allegedly" happened, all I mean is that I didn't see it myself, but I heard tell about it from someone else. The word doesn't imply that there was a witness or evidence or anything like that. In fact it is literally there to make it clear that whatever is being said is second hand information at best.
I'd rather someone say "Tom and Joanna were allegedly kissing in the copy room" if they did not actually see the event with their own eyes.
"Tom and Joanna were kissing in the copy room" without "allegedly" sounds like they know for sure that it happened, which implies that they witnessed it or have proof.
So I think "allegedly" has very nearly the opposite effect of what you said.