I had a (sort of) similar experience after getting my bikes out of the shop. I took 2 bikes in to do a Fit Kit session, and get them both set up properly. They were great in the shop, and the next day, I decided to take one of them for the commute into work. When I started out, everything was fine, but about 8-9 miles into the ride, it just felt 'funny.' I looked down and thought - my seat feels awfully low....and at the next intersection, I checked and realized that my seat had worked itself all the way down to the stops.....I looked at the seat collar, and only one of the 2 bolts had been tightened (it's got one on each side). I fixed it when I got to work, but I had to wait until I got home to check it against the height of the other saddle to make sure it fit properly....
I thought about saying something to the shop, but it's fine now, so I don't know if I will or not.
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2020 Giant Defy 2
Fuji Cross Pro
Scattante 650-R trainer bike
'06 Kona Jake the Snake