Hmm.
I have this question too.
I did some testing with a number of transitions (20.8.3211). Many transitions, when put at the end of a clip work as expected: they do their bit as if the next clip is black (dark). Some transitions, like a number in Geometric do not work at all, when put at the end without a following clip. They seem to work fine at the beginning as you mention.
I have also tried to trick the program: put a clip behind the clip put a transition to overlap both and then drop the opacity of the 2nd clip to 0. Then again the transition does not show. Also putting a transition to overlap a clip and a second clip that has a zeroe opacity, does not work either.....
Cutting the end of the clip into 2 pieces and put a transition over it works, but gives unexpected results to say the least.
And for a number of the transitions I see that when the second clip opacity is almost 0 for the duration of the transition in the second clip the second clip is visible and sometimes even overexposed.
I think that an error report should be made (and I have). Unless I oversee something?