It pauses because the watch starts recording heart rate almost constantly while you are in a workout, while it takes reading once every 4-5 minutes most of the time normally. It doesn't use heart rate as a clue that you may have started a workout - it uses arm motion, and (probably) checks location services to see if you are moving in a way that suggests that you may be walking. But once you start the workout and it tries to read heart rate constantly and it thinks that you don't have a heart rate, it pauses (probably assuming that you maybe you took the watch off of your wrist after forgetting to pause or end the workout.)

That's just the way it works. I know you think it should work otherwise... but it doesn't. Apple isn't likely to change it unless enough people suggest the change, so use the Feedback form to make a suggestion.

Have you tried using the watch on your other wrist?

It’s more like a physics problem than a software problem. Imagine if Apple decreses the sensitivity on wrist detection to make the Apple watch experience better for tattoed users, so they decide to ignore some instances when the light emitted by the sensors can’t provide enough feedback because they are absorbed by the dark spots. That would overall make the safety features of the watch worse, for example in theory someone could just snatch the watch off you on the street, then fasten it on their own wrist and carry on with it, using ApplePay and such… so overall it’s a difficult problem to handle, it’s no wonder Apple still could not address it after releasing 9 generations of watches. It’s not because they are lazy to release a simple software patch. You can influence them to maybe shift priorities and work on this issue more by submitting your feedback, that’s about all you can do right now unless you are willing to remove or lighten the tattoo.