I believe, it's a US thing. This is a quote from the official Dietary Guidelines for Americans (DGA):

Other Vegetables: All other fresh, frozen, and canned vegetables, cooked or raw: for example, asparagus, avocado, bamboo shoots, beets, bitter melon, Brussels sprouts, cabbage (green, red, napa, savoy), cactus pads (nopales), cauliflower, celery, chayote (mirliton), cucumber, eggplant, green beans, kohlrabi, luffa, mushrooms, okra, onions, radish, rutabaga, seaweed, snow peas, summer squash, tomatillos, and turnips.

Source: https://www.dietaryguidelines.gov/sites/default/files/2021-03/Dietary_Guidelines_for_Americans-2020-2025.pdf (page 28)

I've read elsewhere that the reason for the DGA to conflate them, is because mushrooms have comparable nutrients to vegetables. So, from a dietary and regulatory viewpoint, it makes some amount of sense. But yeah, I feel like you could have just had a category "vegetables & mushrooms".