I also have had the problem in the past, so I quit using iCalendar and do it manually.
The newest problem is the PULSE app - I have been getting bookings in fully booked units because using the app to block dates (according to the help center) can not be trusted. Now I have screaming people who have booked - why are they screaming? Because bdc holds their payment for 3 days. Such a mess.
BDC is a bit of a dinosaur in the business. I have not been able to decide if they are a blessing or if I should just drop them. Customer support is horrendous - they simply read from screens and do not understand the issue from a host perspective. All of this when in return we are expected, as hosts, to uphold an impossible standard including perfect reviews/feedback, fast responses, best policies and best prices - wouldn't it be nice if booking.com also applied these standards to themselves?!
Good luck in however you decide to move forward.