@Teslin0
I totally understand your frustration. I also do an in-person check in and have to organise my work around it, so this would not be okay to me either.
I don't know when it was that you were last hosting but, no, I don't think this is something new (although I do think it's more common with instant book guests, as those are often the people who don't see any need to communicate with the host).
I started hosting in my current house in 2016 and often experienced guests like yours and other issues with check in times. Even though they were not the majority, thankfully, it was common enough to drive me mad. See: https://community.withairbnb.com/t5/Hosting/How-can-I-stop-guests-ignoring-the-check-in-time/m-p/304...
The only thing I found helped was to be crystal clear and strict about it on my listings. I don't want to seem inflexible, but I'd rather set the ground rules and then make exceptions for the polite, communicative guests, than the other way around.
So, now my house rules state that guests must confirm their check in time in advance. I also message all guests asking for this prior to accepting their booking (I turned off IB months ago, but this was also in my pre-booking message and all IB guests were asked immediately if they didn't answer that question).
I know that not all guests will know their check in time when they book, so the rules say either when booking, or at least a week in advance. In reality, I don't stick to that week in advance rule, but I just find it helps a great deal to make it clear that I need to know.
The point is, they need to acknowledge the question, even if it's just to say, "I am not sure yet as I haven't booked my flight, but I'll make sure to let you know as soon as I do." If they do not acknowledge the question within the 24 hour booking window, I will decline their reservation.
I will check in on them a few days in advance of their stay to ask again if they haven't told me. It might seem pushy, but I don't care. Guests who have no consideration for others and their time are not guests I want to host.