I have never heard of this bug, so I wonder if this is one more example of an Adobe support guy who just says something so he does not have to admit that he doesn't know how to help you. Unfortunately, that happens quite often. I would try to upgrade your old catalog again. You should still have that because upgrading creates a copy.
The senior technician did work on the computer and saw the issue. All that has been tried failed. Creating a new catalos and importing from the original folder does ot work. It brings a long list of keywords with no images using them. Only images tagged after the update have keywords. It seem LR recognizes the tags but does not see the tags attached any image
Adobe recognized this is a bug they are still working on a fix ( or at least I have been told)
If it is a bug they are working on, then this must be one of the rarest bugs in the history of Lightroom. I have not heard or seen any other report about it, which is very odd, because anyone upgrading a catalog would immediately see it if it happened to them too.