Connect stole 24000 steps when I switched activity tracker

It might be too late, but did you try to switch back and sync again? Depending on how you do it, Connect may pick up just the steps showng on the new tracker. That's why it's usually recommended to do the switch first thing in the morning.

That being said, I wonder if there has been a switch in the way Connect has worked until now. It used to be that if you had 2,000 steps, that total would remain until the second one caught up. No new steps would be recorded until the second tracker of the day hit 2,001 steps. Now it seems to take the value from the band directly.

I did try reverting to the previous tracker and resyncing - but not until the next morning when I discovered the steps were missing.

Switching first thing in the morning is not a viable option for me. My first activity starts shortly after 5 AM, and my early mornings are pretty hectic. Except on Fridays I usually reach my 15000 step goal before breakfast.

This evening will be my next 920XT swap, and if I lose steps again I guess I'll assume Connect has changed. If so I may go back to using my Vivofit for activity tracking. Then I'd also revert to using one of my 910XTs for activities - the 910XT alerts are strong enough for me to detect while swimming, but the 920XT alerts are too feeble.

Actually, given that I just ordered an InBody wrist band that does activity tracking, HRM, and body composition measurements all in one device, I might abandon Garmin for activity tracking and use Garmin devices and Connect just for swimming and other discrete activities.

Update: It's the next morning. I swapped trackers last night, and my step total for yesterday was zeroed out when the new tracker synced. My steps daily summary for yesterday, which you may be able to view at https://connect.garmin.com/modern/daily-summary/stpeters00/2015-12-18/steps shows thousands of steps distributed throughout the day but says they total only 43 steps.

In other words, Connect isn't even internally self-consistent. Connect really needs a consistency checking procedure, because this isn't the only place it's not self-consistent. Some of my activities show up with a moving time longer than the total time. Consistency checks can provide big hints to developers about where hidden bugs lie, especially round-off bugs.