Either the propagation path changed (that’s impossible) or the behavior of the satellite antenna changed… any comments please from the sattelite gurus?
The satellite is stabilized and I don't think anything can change.
What can change, in my opinion, is the path of your uplink signal due to propagation (rotation) ... but not only.
Your antenna is metal, exposed to the sun. The metal exposed to heat deforms. Slightly but deformed. It would be interesting if you have the opportunity to repeat the tests at night, and in broad daylight to see if there are variations.
P.S.
in any case 3 dB are many ...