that sounds like you might've soldered the connections on the opposite side of the connector. the buttons are like pins 5-8 or something like that. and the hex inverter goes to pin 6 and 7 or 5 and 6 from the end. cant really remember the exact pins.

check the lines above the screen header, check for bad solderjoints or bridges between them

Need help guys, I installed a hex logic inverter it works great, but now sometimes the buttons don't work, they do the wrong thing or the randomly press on there own.

Any ideas ?

Cheers in advance