Wrong Stroke Detected

The device will only detect what it believes to be the stroke you are doing. Clearly your use of paddles has it confused. Unless you are used to paddles they will tend to slow your stroke rate and possible change you style. This may well be sufficient to confuse the poor thing. What is your stroke rate using paddles?

I recently did 4km with paddles at a stroke rate of around 28 and didn't lose a stroke or have it misinterpreted (920) https://connect.garmin.com/modern/activity/1601828531/2

Try to tighten the watch strap. Sometimes it helps when watch is fixed well.

I have a Fenix 3 HR and two Forerunner 920XTs. Until they died I recently had two Forerunner 735XTs. I've swum almost 120 miles, all freestyle, just this year with these devices, plus many more miles in the past.

The 920s and 735s almost always detected my freestyle correctly, in spite of my partially crippled left arm and lopsided stroke. The 920s in particular almost never get a length wrong. I estimate they detect about 1 length wrong (as backstroke) per hundred miles.

My Fenix, OTOH, does much worse. In some mile+ swims, it detects about half the 25-yard lengths as backstroke.

I don't fault the Fenix. The core problem is my lopsided stroke, and it's remarkable that any of these devices ever correctly detect what I'm trying to do. However, my results do show that either there is some basic difference between how Forerunners and Fenixes detect stroke type, or (more likely I think) there are minor variations in hardware performance within any watch type and my particular Fenix just happens to be on the less-forgiving side. In other words another Fenix might do as well as my Forerunners.

One thing to try would be to wear the watch on the other wrist.

The 920 is indeed extremely good at detecting stroke type.

However, to my previously reported experience with Forerunners and a fenix 3, I can now add that a fenix 5 is as poor at detecting my freestyle as is my fenix 3. Whereas my Forerunners almost never report a wrong stoke type, my fenixes report an incorrect type more often than a correct type for some swims.

Because I never swim anything but freestyle, the stroke type detection feature is sufficiently unimportant that I now simply turn it off in the watch settings.

As for wearing the watch on my other arm, I have to wear it on my partially crippled arm so I can operate the buttons with my good hand.

I still find it remarkable how well my 920s detect my stroke type. However, although my stroke feels lop-sided to me, I'm told it looks smooth and graceful, so perhaps there's some muscle memory left over from my youth (52 years ago). Maybe I should be less impressed at the 920s getting my stroke type right and more annoyed at my fenixes getting it wrong.