Garmin has recently released a new version of the firmware for Vivoactive 3: the 5.20, in the ChangeLog, we can see:
Change Log 5.20
Fixed golf activities not recording distance
Added support for abnormal heart rate alerts
Fixed UI changing pages while swimming
The last element hides the deactivation of the touch screen for swimming-type activities making all third-party applications completely unusable, of course, the Apnea versions are affected by this problem.
Note in passing the complete lack of communication from Garmin who moreover recognizes it (see Vivoactive 3 Touch Screen Issue) and is unfortunately usual.
For Apnea, I found a temporary solution by changing the type of activity to "Other", a next version will lock the screen.
These versions are exclusively for Vivoactive 3, so update only if you have a Vivoactive 3.