I had already raised the same issue in a separate discussion, but after a bit of research (digging), I decided to start a new one because I am now more confident that it is not a simple bug but a systematic compatibility issue.
It looks like WRI has gradually developed paclet tools (including documentation tools) for years. With Mathematica 12.2, the paclet tools seem to have become quite "mature". It has a complete new packages bundle PacletTools under SystemFiles/Components, and it includes DocumentationBuild. I guess it might enable us to build documentation files even without Workbench (for small-scale projects).
Two unfortunate things:
Obviously, the compatibility with the current public version of Workbench is broken. For example, when you try to build your documentation, Workbench tries to load DocumentationAssets, which is intended for WRI insiders (according to PacletTools, it should never happen). It is said that they are internally much advanced version of Workbench inside WRI and the internal version may be compatible. I hope they release an updated public version which is compatible with 12.2.
The documentation of PacletTools (and included DocumentationBuild) is poor. You cannot apply those new tools to the documentation files generated with Workbench; many deprecated functions and so on. I could not yet figure out how to work around the problems.