First of all the issue has nothing to do with diving.
Since the MK3i is based on Epix (gen 2)/Fenix 7, I write about my issue here also, since the MK3 thread is not "moving" so much and I also understand that most people there got the MK3 mostly for its diving capabilities, and not so much for other sports, so they may have not met the problem (actually another member there has the same issue after the update)
Maybe some of you have encounter something similar on your watches and have some insights or suggestions.
So bare with me and read on :)
Everything was working normal on my workouts (running and walking), until I updated to 19.34.
After that, the watch does not produce any training effect data (primary benefit) when I have a chest strap on. Or should I say it's a hit or miss (but mostly miss) case to whether it will work correctly or not. Sometimes it does, but mostly it doesn't.
For more than a month we exchange mails with Garmin support but they have not come up with anything really. Τhe only essential comment on one of the mails, was that they observed some minor micro-interruptions in the connection with the chest strap and this might have skewed the training effect data and maybe I should try to change the battery on chest strap. However on all activities (even the ones without training effect), my HR graphs are just fine.
All the data are fine (heart rates, respiration, etc) but NO training effect is produced when I save the activity. Actually RUNALYZE has no problem showing/producing these data even when the watch and connect do not.
Things I've tried either because I was told by Garmin to do, or by myself.
1.Hard reset
2.Reset to factory defaults.
3.Changed battery to HRM Pro+ (my main chest strap)
4.Bought and tried a different chest strap
5.Change of clothing (in case of static)
6.Unpairing and re-pairing the straps.
Nothing helped. The same behavior.
If I use the watch's optical, it is ok. If I put on a chest strap then things go south.
The weirdest thing is this. On my today's run with the second new chest strap, I was watching onε of my data screens which I had set to TRAINING EFFECT. Up until the last second before I ended my activity and saved it, the data screen was showing training effect just fine. After saving, all data are there except TE !!! 
https://connect.garmin.com/modern/activity/17919318626
And the whole problem is that a lot of the metrics depend on that TE metric.
So if any of you have any suggestions, or seen something similar or can brainstorm about it, I am all ears.
I remind you that epix and Mk3 are basically the same software, UI, etc.
