I have never gotten a corn seed from the unknown seeds. Am I missing something? Was there a bug? Mod version is 1.5.2 for 1.16.5.

After our code review and actual testing, we found no problems and we were able to get corn properly. You can check your configuration file, which is located in {minecraft}/config/crockpot-common.toml, and check the unknownCropsList item for corn seeds (crockpot:corn_seeds).