Apple Watch shouldn't change the watch face on it's own. I would say you are likely swiping between the watch faces throughout the day without realising it, I do this quite often myself.
You could avoid this by deleting all other watch faces from your watch and only keeping the one you want to use. The deleted watch faces can still be added back easily at a later date if you desire.
To remove the watch faces you aren't using:
- Go to your current watch face, then firmly press the display.
- Swipe left or right to the watch face that you want to remove.
- Swipe up and tap Remove.
See Change the watch face on your Apple Watch – Apple Support for more information on managing watch faces on your Apple Watch.