By default, altitude is measured with FusedAlti during recordings that use GPS and during navigation. When GPS is switched off, altitude is measured with the barometric sensor.
If you do not want to use FusedAlti in altitude measurement, set your reference value with altitude or sea level pressure.
You can search for a new reference value with FusedAlti by activating it in the options menu under ALTI-BARO » Reference » FusedAlti.
In good conditions it takes 4-12 minutes for FusedAlti to find a reference value. During that time, Suunto Traverse shows barometric altitude with '~' before the reading to indicate that the altitude may not be correct.