Weird one?!?! I normally run my 330 250 Yamahas at 2 bars for best efficiency at speed and use the trim tabs to level the boat port/starboard. You said the trim tabs were "calibrated". If so, the following may have already been done.
The trim sensors on the engines are spring loaded mechanical device and have been known to stick and show no trim on the gauges when, indeed, you are trimmed (depending on your motors, this sensor may now be electronic). You might manually move the sensor mechanisms to see if they are moving freely. If sticking, a shot of silicone lubricant will free them up.
Another thought is to move the engines up and down in your slip and watch the trim readings on your gauges to determine if trim sensors are working properly.
Admittedly, a longshot - but easy to check.