Could it be that you searched TheTVDB with the “special characters removed” (no colon) title (or original filename title) and not the actual title (with colon) from the metadata? Or do you search both and first one wins?
I ask because almost all of the shows that don’t have much guide metadata are PBS OTA sources and the guide data that does get put into TheTVDB, IMDB, etc. comes from fans. PBS isn’t like the networks or studios that have marketing teams that make sure that their show guide data is widely published and populated.
Then there’s the problem of “erased” and “clean-washed” content (along with the metadata) in the aftermath of scandal, such as the cooking shows from Martha Stewart, Paula Deen, Mario Batali, John Besh, 15 seasons of Chris Kimball’s ATK/Cook’s Country, etc.
PBS/APB also doesn’t have a good track record for metadata for older shows, particularly on their Create channel, because the shows revolve around a particular personality/chef/star and the marketing was left to the indpendent producers back before streaming and getting your episode metadata into TheTVDB or IMDB (even less after Amazon took over and monetized it) became vital to marketing the DVDs or streaming them on-demand or curating home libraries the way they are now. For example, old shows from John Folse, Steven Reichlen (Project Smoke, BBQ-U, etc.) or paywalled shows from Food Network or HGTV.