This is abasic first question, but are you using the correct workout type in the Workout app on the Watch? There are indoor runs and outdoor runs

Second basic question. Do you have your vital stats set correctly? That messed up some of my metrics before

After that you might try some indoor running on an indoor track that way you can know for sure the ground you are covering is correct there. That might also give a clue about something we aren’t even thinking of

Lastly, you might check your form running on the treadmill. Set up your iPhone to record your run from the side. You might be doing something odd that you don’t feel.