Update Plugin on Post operation is not updating the form

HI,

suppose, I'm having a plugin on Update of Account with Post-operation stage and a Pre Image.

Requirement is when I update field1 on the form, plugin needs to update field2 with a value from Pre image.  But it is not updating the field2 with preimage field value.

When I use Pre-operation stage for the same, it is working as expected. Field2 is getting updated with the value from pre image.

So, why it is not updating the value from pre image on post-operation.

(In tracing I can see the field2 updated value, but form is not getting updated even after refreshing also)