I use my Fitbit Charge 5 to log both Workout and Walk exercises. I've done this for a couple of years.

In my gym workouts, my heart rate has commonly surged to 120 to 140, both during the workout and occasionally up to 10 minutes after, while I'm resting. I've been a bit suspicious about this because my workout is not that rigorous (and I don't feel any different during the surge). I suspected AF but the Charge 5's ECG function did not detect AF when I tested it during my rest period. I also tested my pulse with a pulse oximeter which typically showed an HR of 80 to 90 (while the Charge 5 was showing 130). (FWIW, I had AF a couple of years ago after cardiac surgery.)

I've also logged my walk using the Workout mode. My typical walks are 10 to 20 mins and reasonably brisk, but the HR is always under 110. Using Workout mode it showed HR again in the 120 to 140 range. It felt no different from a normal walk.

From this it seems obvious that the measurement of HR is different in the two modes, and probably incorrect in Workout mode.

Does this sound like AFIB? Any other suggestions?

No, doesn't sound like afib at all. Sounds like an inaccurate recording device. My wife has a fitbit and loves it, but I've found it be very unreliable.

If you really want to know, buy a Kardia. Much cheaper than an Apple Watch and more reliable. You don't need to sign up for the subscription and you don't need the more expensive 6L model. Or, just practice feeling your pulse. That's entirely free and very reliable. If what you feel is a steady beat, a rhythm with consistent timing no matter how fast, it's not afib.

I have a Fitbit and a Kardia and agree with Carey— the Fitbit isn’t very accurate and there is also a lag in reporting your HR. I have a pacemaker which is set to pace me anytime my HR is less than 70 yet my Fitbit often tells me the my HR is in the low 60s—go figure. I use the Fitbit for other things like steps, which it is pretty good at.

Thanks for responses. I was starting to draw the same conclusion. Trouble with the Kardia is that it doesn't show the ECG during exercise.

Also my pulse oximeter 'shows' the pulse action and this didn't seem to showing the high pulse I'd expect. (My pulse is relatively hard to take by hand.)

I'm now fiddling around with a Polar H10 strap and an ECG app. Been getting some interesting results but nothing that looks like AF. Certainly arrhythmia, but I knew that before. I guess that's what's throwing the Fitbit.