"After a few days" has meaning.
The principles of altimeter calibration in tourist devices are the same, in the 66 they are similar to those in the 67, the problem is that the 66 did not acquire full satellite acquisition immediately after launch,
when activity recording was automatically turned on, an incorrect GPS altitude was assumed.
incorrect altitude = incorrect altimeter calibration.
Solution:
Update the EPO.BIN file by connecting to Garmin Connect or Garmin express.
Turn on activity logging manually after complete satellite acquisition.