Setting my BCH workers to PPLNS from PPS is not working on Antpool

I have tried to set my two BCH workers (one S9 and one S7) from PPS+ method to PPLNS, but it has not changed, it is not possible.  (under settings -> payment section)
I have tried to switch back and again to PPLNS several times, but it is not working - on the earnings page I can still see entries under the PPS+ method.
However in the past I could switch to PPLNS.

how can I switch?
thank you in advance,