"The human brain is an incredible pattern matching machine" - Jeff Bezos
That is the problem. Cable break in does not exist. Audio is AC, so degaussing residual DC in a dielectric would be nullified in a couple of milliseconds of play.
However if you stop flexing your cables and just leave them alone the micro-fractures from handling the face centered cubic copper wire structure might join back somewhat. However I doubt the difference is noticeable.
Most likely, running a signal for a while might also strengthen a tentatively poor connection. Since the change you noticed should not happen if your cables were OK in the first place, you should really get some measuring equipment for a small fraction of your investment. Meanwhile use some antioxidating connectivity fluid. It is a good practice to do that anyway.