it happens to me too, but after changing the setting it's fine now. the watch is being accessed as a drive, this would automatically stop and save activities. You can go to Setting > System> USB mode …
If you are charging your watch during a long activity, before you start that activity, you will need to put your watch into Garmin USB Mode.
Long hold your middle left menu button on your watch to go your…
If you have a newer watch the menu items may have changed. On fenix 5, from seeing the time of day, hold down the middle button on left until it changes, then hit the top left button to scroll up one click…
Not sure if you have resolved this but I have had similar problems with my 5s. During a recent long bike packing trip I needed to recharge my watch and while charging it ended the activity, the first time I wondered if it was as battery power had dropped below 10%, but the following day it did the same twice, first time battery was low but the second time the watch was charging on my handlebars so I could keep an eye on it, battery level at 46% and it just bleeped and completed the activity. Interested to know if you managed to resolve?
I feel like there is something happening with the way the pins connect or disconnect with the charger, so my tip for charging is connect the cable to watch and then plug USB into charger, and for disconnect, pull the usb from the charger first then disconnect from watch.
I have also messed with the watch USB setting; in Garmin mode you get the extra protection when connecting to charger if it somehow thinks it's a PC it asks a question before saving the activity and otherwise just starts charging.
it happens to me too, but after changing the setting it's fine now. the watch is being accessed as a drive, this would automatically stop and save activities. You can go to Setting > System> USB mode > change this to Garmin if it's on Mass Storage. The device will now have to ask u to use Mass storage mode if u say no, it will continue to track the activity without doing it for you automatically.
If you are charging your watch during a long activity, before you start that activity, you will need to put your watch into Garmin USB Mode.
Long hold your middle left menu button on your watch to go your menus. Then you will select Settings > System > USB Mode and change your default MTP (protocol used to be seen when plugged into a computer USB port) to Garmin.
Be sure to change your watch back to MTP if using your computer with your watch. IE: Using Garmin Express, accessing files etc.
How do you do this? I can’t seem to figure it out on the watch itself and when I go to settings on the app (on my phone) I can not find ‘system’ or ‘usb mode’. Very confused.
If you have a newer watch the menu items may have changed. On fenix 5, from seeing the time of day, hold down the middle button on left until it changes, then hit the top left button to scroll up one click to Settings then pick up the Garmin - Chris steps.
Thank you!
You will make the change on your watch. carleson gave more information. I have also updated my blue boxed official comment to be more detailed to help avoid confusion.