I am booked on BA 277 LHR-HYD in April 2022. I needed to book additional tickets on the same flight as mine for family members. However, the flight is no longer bookable.

When I check, the flight is no longer bookable from March 27, 2022 to June 30, 2022. It is bookable prior to and after these dates. However:

1) My existing reservation in April 2022 shows as active and not canceled.
2) ExpertFlyer continues to show the flights as active with no schedule change and seats available.
3) The return flight BA 276 HYD-LHR is bookable throughout April 2022 - June 2022. There is only one flight a day from LHR-HYD and return so if the return flight is active, there needs to be LHR-HYD operating to turnaround the plane.

Why would the BA 277 LHR-HYD route not be bookable from April 2022 - June 2022 despite the above?

Thank you.