If I'm not mistaken, the lcd display is initialized before the mcu even speaks to the synth over i2c. So the two rows should disappear (and probably display text) before the i2c synth stuff begins. So the 27MHz crystal is irrelevant during this phase. The mcu has it's own 20MHz crystal to run, so it does not need anything from the synth, as is clear when you power up the board without the synth.

Search on the synth board for the problem first. I suspect there's something wrong with the level shifters (Q1,Q2) or the i2c pins (4,5) on the Si-chip (measure at R3,R4). They should both idle at 3.3V at the chip side and 5V at the pinheader to the QCU board.

I'm not so familiar with the Si-chip as to know what it does when it's powered up without a controller talking to it, perhaps in the datasheet. Still this does not help too much if the problem lies in the i2c pins and level shifters.