Perhaps it would be better to not do the Serial in the I2C receive event. Make a copy (buffer) in the I2C event and set a flag (got_i2c) so the main loop can see it and copy the buffer to the Serial.
My guess is the serial buffer fills up (e.g. more than 32 bytes) and then starts blocking while the I2C receive event is running. The TWI code that is used for Wire starts clock stretching before running the receive callback, so I would guess that having a blocking serial during a receive event would be a disaster.