Same problem here, but I wonder me if it's 100% hardware ?
When it happens (trying aaa eee bbb aaa eee bbb sequence with normal press) dmesg says (when missing a letter) :
"aw9523b_irq_handler: enter" two or three times and doesn't go with "key press", which is related (if I understand correctly) to i2c GPIO expander
But yes when I press very hard it doesn't happen, either when I write slooooowly, wich could be the same here...
Not sure at this point but really annoying