The category field in the generated metadata.json file as of current corresponds to a string value, however there are doujins out there that can contain multiple categories, so the corresponding value should be a list, like other fields. Here are some examples that you can use to verify: 96684, 121732, 132586, 115122, 14651. Currently when it encounters doujins like these, the resulting value for the category field is a comma separated string. The information is preserved thankfully.
The category field in the generated metadata.json file as of current corresponds to a string value, however there are doujins out there that can contain multiple categories, so the corresponding value should be a list, like other fields. Here are some examples that you can use to verify: 96684, 121732, 132586, 115122, 14651. Currently when it encounters doujins like these, the resulting value for the category field is a comma separated string. The information is preserved thankfully.