It could just be the nature of the feedback time you have setup. Because it ping-pongs one side just may be seeing a repeat or two more than the other side. Over time this could build up to a difference in levels.
Although, if you are sending the signal using a “post-pan” send and your track isn’t dead centre then it’s left/right balance would be reflected in the plugin as well.
Another thing to consider could also be the Analog mode you have selected. Since the purpose of that feature in H-Delay is to offer different analogue delay emulations, the levels of the left and right channel just may have been slightly different due to the imperfections of the modelled equipment.
Is this how it appears on your meters or do you hear one side louder than the other. If the answer is the latter it may just be a “perceptual” thing. There are plenty of ways for the ear and the brain can be tricked this could be one of them.