MicrosoftThank you SO MUCH, Shreya_Ganguly , for letting us know.
I've found that there is one exception to the following truth:
for use across all sites in the organization
Namely: This does not work in a site used for a Private Channel.
If you are interested in why, and have access, there is an open ticket with MS SUPPORT - [Case #:30693827] Error adding Content Type to Private Channel Site
I've summarized here:
Bug? Add-PnPContentTypesFromContentTypeHub doesn't work in Private Channel Site · Discussion #1770 · pnp/powershell (github.com)
