onInit with HD44780

B6 and B7 on the Pico are pins used for Serial1 - which is where the console moves by default.

I guess that might be affecting things - while they shouldn't transmit anything for ~1 second after startup (by which time you changed the pins around anyway), I guess it's possible that their startup values confuse I2C on the display and it needs a timeout to recover.

I'd hope that the following - as suggested above - might work:

function onInit() {
  I2C1.setup({scl:B6, sda:B7});
  setTimeout( function() {
    var lcd = require("HD44780").connectI2C(I2C1);
    lcd.clear();
    lcd.print( "Hello World!" );
  }, 2000 );
}

But the other option is just to use some different pins - it's bad luck - those two are literally the only ones on the board that get used for something else by default :)