You are most likely at the wrong den, as the two are very close to each other.
Ditto only appears in the den as a Rare encounter (i.e. a Purple Beam), and the Common variant of the same den only produces various Normal-Type Pokemon. You are probably not at the den you think you are if you encountered Bronzong, a Pokemon that only shows up in Steel-Type dens.
If it's any consolation, if you were in the Stony Wilderness, you were most likely at the den where Gigantamax Copperajah spawns, as it is the only Steel-Type den in that area.
(Just FYI, the Ditto Den is in the Stony Wilderness, and it is Den 66 on Serebii and Den 59-Rare on Raidfinder.)