Similar situation, sort of. I booked about July 2022 for a trip to Italy April 30 2023. I've been with Delta 20+ years and 'always' fly Comfort+ bc that's my 'comfort zone'. Don't remember the aircraft type, may have been A330 or B767 but there was a rather large section for Comfort+, i counted about 30 seats. At the time of booking in 2022, not only was pretty much the entire aircraft EMPTY, but the ENTIRE Comfort+ section had an "X" in each seat and when hovering over each seat the pop up said "unavailable". What ? There was no way I could book in that section so I went ahead and chose main cabin thinking i could change later. Nope. Engaged in 2 online chats, those agents didn't know why they had an "X". I called 3 times...2 of the agents said "hmmm, that's weird, never seen that before". Third agent said they are X'd and noted as "unavailable" because they are held for crew, etc...possibly families or persons with disabilities. I have NEVER seen these X's before but I get it for the reasons they stated but to have an entire section as unavailable ?
Called 1 more time to try & move up, the agent then told me that the Domestic portion is Main Cabin and the Int'l portion is Comfort+. Said it's "always" been that way. Nope. But..what they ARE doing now that is fairly recent is the chart that you use online to choose your flight times and cabin type has changed. The headings remained the same but you have to be careful in choosing your desired flight time because SOME, even though you are within the Comfort+ column, will state "mixed cabin" or will spell out "Main Cabin / Comfort+". Very annoying. Apparently I didn't see that when I booked. The agent was very nice and ended up moving me to a Preferred Seat and said that's the best she could do, which I appreciated and I'll be more watchful when booking next time.