this is not a bug.
there is no bug as far as i’m aware.
it’s just that belial ambushes are very rare. I have done over 600 boss runs (everything minus harbinger) and the average spawn ratio was about 1 in 20 runs.
the 80% chance increase is based on the probably 3% base chance, so it brings it to about 5% chance of spawn. Hence 1 in about 20 runs.
I thought this was a bug as well before i started doing a crapton of boss runs non-stop. Just this memorial weekend i’ve done over 300