In general, the closer to integers the values in CKKS you want to switch to FHEW, the better. During switching, the rounding is done over ciphertexts, as FHEW supports only integer messages, so it is not exact and some errors are to be expected. While in CKKS, the precision and “separation” between the message and the error is dictated by scaleModSize, in FHEW, this separation is dictated by Q/P, which is much smaller. My suggestion, if your application allows, is either to scale up (e.g., by 10) the messages before switching, or to decrease the plaintext modulus (or increase the FHEW ciphertext modulus, but that is limited by the security).
I will also look more into it to see whether there is a bug.