None of the challenge boards are random. Each time you play a challenge the board always starts the same. Karakoth first challenge at 5 stars always has 2 easy 4 yellow matches for example.
Alas, this appears to affect console users only. Certainly didn’t work when I tried it on iOS just now.
Edit: I am pretty sure what you’re seeing is not the intended behavior.
Yep PC here and checked random boards.
The beginning board is not random, but the gems that fall seem to have a set number of different possibilities