I'd love to see more data.
I've seen examples, which are often summarized as "after the update the keyword is directly associated to a category by google, so there is now a lot more competition even if the otehr GBPs are not targetting the keyword".
I also suspect that certain element are just getting refreshed without actual algorithm changes per se, causing for example businesses with no recent reviews who had long enjoyed great success to abruptly tumble out of the threepack.