I think everything is correct. What I think is wrong is the percentage associated with chances to hit based on the modifiers. I think its actually lower. Not sure what calculation there are using but its off a bit. 50% to hit, is not really 50% to hit but more like 25%.
Can you give an example to support this? If possible, linking a screenshot of the percentage, your modifiers, and the enemy AC?
It's pretty simple to convert a # needed on the d20 (even considering advantage/disadvantage) to a percentage. I'd be shocked if there was an error in that calculation; it's much more likely that Larian implemented some weighted/weird rng than this.
AFAIK, the only percentage displayed incorrectly is when the games says you have a 100% chance to hit. This is due to rounding 0.9975% to 100%, as opposed to an actual calculation error...
