wrote:

Were you being vectored to the final approach course? If so, you need
to be established on the FAC at least two miles prior to the FAP. If
you're not established two miles out from the FAP, the approach mode
will not activate.

It will unless the intercept angle is really ugly. Garmin doesn't define
the limits. The approach mode will also drop out inside the FAF if you
turn significantly away from the approach track.