As the slip concept is being phased out, this happening is becoming common when zones aren't able to draw out a dedicated slot. The same thing happened with 18181/82 and 28181/82
18181/82 now runs 4 days instead of daily and 28181/82 runs 2 days instead of daily.
Zones were able to carve out a new slot for 18189/90 although which is not much appealing to the public.
There...
more... are many such examples where the slip concept was broken and days of run reduced. If zones become ready to figure out a new slot, it will be a welcome decision for the public.