Stamets has apparently never been intended to be the "proper" Chief Engineer.
A display in "Choose Your Pain" (DSC) did list him by that title:
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But according to Ted Sullivan, that was "a flub." (Nevertheless, as it was prominently seen onscreen, we might posit that Stamets did act in that capacity during the time when the ship's primary mission was to get the spore drive working.)
-MMoM![]()
Or like the early voyages of the Enterprise-D, the Discovery has multiple chief engineers and assistant chiefs. Stamets was chief engineer of the Spore Drive. The other chief is the one for the warp drive.