For the last 2 weeks, all my diggings with over 70% probability of getting an uncommon artifact turned into common ones. The only digs that gave me uncommon ones are that with over 80% probability.

I found it pretty weird because in the past, if I digged 3 artifacts with 70% probability, I got at least 2 uncommon ones. Now I get none.

My friend, you have discovered how probability works.

Humans are very good at finding patterns where there are none.

Your sample set is too small to make any meaningful conclusions, for a realistic comparison of the actual results to the expected, you would need more like 100k or a million samples, depending on your tolerance for error.

Am I… a goblin wizard?

The topper w/probability diggings is each dig stands alone. That is, you can dig 100 digs with the 3% chance of an Epic thinking you'll get at least one Epic, but never get any Epic.