It’s not clear to me what your goal here is when pausing. Do you want to pause the whole pipeline or only the branch with the appsink? And why?
The problem in your application however is probably the following: You’re setting the play_sink to READY state as the first step, and afterwards the source to PAUSED state. What will happen here in most cases is that the play_sink will receive another frame in between and then the sink will return an error (FLUSHING), which will silently shut down the whole pipeline. The opposite problem happens when unpausing: you first start the source and then the play_sink, which would likely let a frame pass through to the play_sink before it can actually accept a frame, leading to the same situation as before.
Depending on what the goal here is, changing only the source state or the whole pipeline state is more likely what you want to do. Or maybe you simply want to ignore new frames in the appsink callback and keep everything running?