I think I can relate to never being able to score something significant on slots with I was given some free spins. I am not a big slot player myself, but when I get the chance to make use of some spins given as promotion I go for it and see if I have got some luck.
I don't think you should not overthink about it, it is statistically possible for anyone who plays long enough on slots to experiment something like that. It is not as there is something fishy going on behind the curtains of the casino or the game provided by a third party.

Someone paranoid enough would have guessed there is some rig behind that "pattern", but most of the time it is not the case.