Chinese employer wants me to resign after their client left. Is this legal?

I’m on a Chinese work permit and residence permit. A Chinese company hired me directly and then assigned me to work remotely for one of their clients. My six month probation period ends on Monday. The client decided not to continue its contract with my employer after the probation period. Now my employer is asking me to sign a resignation letter instead of keeping me employed. They are not saying they will terminate me. They are only asking me to resign. They also do not want to mention in the resignation paperwork that they lost their client or that the client decided not to continue working with them.
Can they legally pressure me to resign in this situation? If I refuse to resign, can they legally terminate me just because they lost their client? Can they cancel my work permit on that basis?
I am currently outside China and plan to return soon. Is it risky to return while this is happening, or is it generally safe to go back and deal with the situation after I arrive?