HR very high (220ish) at start of activity b/f dropping to 'normal'

Has anyone else experienced this?

Wahoo Tickr put on well before ride start (10 to 45min before) but when it pairs with the 1050 and the activity is started, the 1050 displays 200-220 before slowly dropping to 'normal' (100-130) over about 5min. My 830 does not do this.

It's one-way ANT+ communication so the Tickr doesn't know when the 1050 connects or that an activity has started AND I've had the Tickr on for up to 45 min before the ride.

Any ideas?