distortion on transition of bits in FSK
greatly appreciate any pointer!

It appears (and it isn't that obvious so I might be wrong) that you have an input filter circuit that is very highly tuned as per this in your question: -

And if that is so, then what you are seeing (when data changes the FSK frequency from 23.5 kHz to 32.2 kHz) is the tuned circuit &quot;ringing&quot; for far too long. I recommend that you scrap the idea of filtering without damping (the stuff that makes things like this actually work) and use.. er .. damping to prevent the extended unforced oscillations being produced when you change the excitation frequency.
You need to design a better filter system with much lower resonant peaks and damping is your friend in this respect.