5+ adding laps in pool

Why would you need to calibrate the compass? if you are in a swimming pool there should be no need for a compass or GPS. Some usual questions come to mind:

Are you using 'Pool Swim' activity?

What stroke are you swimming?

Are you swimming continuously?

What buttons do you press at the end of each set?

Dr Phil said:

Why would you need to calibrate the compass?

There was some speculation that compass is used to help detect direction change when making a turn in a swimming pool, but even if that's the case, I do not see how calibration could affect it in any way.

And the other questions?

I believe almost all of the answers to your questions are in my original post.  Yes, I'm using pool activity and I press the "lap" button to end and start intervals (after the initial "start").  I will add that since this post, I have swam two more times (3000m and 2050m) and haven't had a recurrence of the error.  I'm not sure if the calibration helped, but something changed and that's the only thing I did...

Thank you, I read that before posting here.  I have had two successful swims since calibrating the compass...

I have the same issue on the Fenix 6:

I swim in a 50m outdoor pool. I swim a very consistent pace each time (I'm trying to improve but pretty much at my max for each length). I do 5 sets of 4x50m intervals with a 2min rest (so 200m then 2min rest and repeat until 1000m swam). 

I've done it 5 times now and every time I get a different distance. It pretty consistently counts too many lengths. It doesn't count too many for every interval instead the first interval will be correct and say 200m. The next then might go up to 300m but I've still only done 4 lengths. And the same for the next and then it normally gets it right again for the last couple. Very frustrating and no way to properly correct it. I can manually amend the distance for the whole swim down but it still thinks my fastest 100m is twice as fast as it actually is. 

iamtomfloyd said:

I've done it 5 times now and every time I get a different distance. It pretty consistently counts too many lengths.

As far as we (ordinary users with no secret inside knowledge from Garmin) know, the watch detects laps by looking for changes in your swim style, which could mean that you have taken a turn. It seems that it may specifically be looking for the glide after a turn.

So if you at any point during the lap will stop swimming for a moment, it may be misinterpreted as a glide, and the watch will think that you turned.

Have you done a manual calibration of the compass?  I don't know why but that seemed to fix the issue for me. https://www8.garmin.com/manuals/webhelp/fenix66s6xpro/EN-US/GUID-4C4297B5-1632-4CC8-98B9-67C6682A3745.html

Are you using the Pool Swim activity? What stroke are you swimming? How fast /100m? Most of the time these issues with length counting come down to swim technique and/or something happening during the swim that affects the length count like stopping mid-way, banging lane ropes etc.