I am experiencing the same problem. Found out about the issue last Friday (5/25) when I received an IB for a date that should have been 'blocked' that I knew wasn't available. A ticket has been opened with AirBnB and I am anxiously awaiting a solution.
As I've been monitoring this issue for several days now, this is what I've seen:
The AUTOMATIC calendar block data feed from VRBO & Homeaway isn't working. Yes, you can do a manual sync but it only blocks for a little bit of time then the dates are again open. The calendar block data feed from booking.com IS working.
Placing a manual block on dates directly through the AirBnB calendar doesn't work either. It'll show blocked for a few minutes then the block disappears.
And yes, I've relinked the calendar feed several times. The blocked dates are pulled in but later disappear and my calendar is again open. Personally I believe there is a system issue where, when the system goes to pull in the data to do the sync, it is overriding any previously placed date blocks which is why a manual calendar sync or manually placed date block doesn't stay "blocked" very long.
My listing is now 'snoozed' awaiting a solution to this issue! Hopefully the more AirBnB hears about this issue the more efforts will be placed on a resolution. Thus far I thought I was the lone voice...