Everything sitting around a while needs time to come back. Happens overnight, happens when shipped, for sure happens when stored away for a long time.
This happens in part because no dielectric (insulator) is perfect. They all absorb some small part of the signal which is then later discharged back into the wire. This is the main reason different dielectrics do sound different. This is also the main reason it can take so long to fully burn in certain components. Over time the dielectric gradually becomes saturated to equilibrium, at which point it becomes stable and the sound improves. Let it sit some time this discharges and we get to go through the same process all over again.
Phono cartridges have this but not to the same extent owing to the extremely thin dielectric. But phono cartridges also have a suspension, typically a little donut of elastomer. Stored at rest the cantilever is at zero load and so the suspension adjusts and conforms to this state. Playing a record VTF causes the cantilever to bend to a new angle. It can take some time for the suspension to fully adapt and equilibrate. This is a main reason we are told to put some hours on before fine tuning VTA. Tracking angle will change with use.
The sound you heard is pretty much what I would expect. Just look at the above factors. Your stored suspension is a little stiff, so not tracking quite as well. If it was stored a very long time the elastomer may be hard and never return to "as new" but that is a bit of a guess, plenty of people using carts for many years still loving them. I would put more hours on it and see.
