Andvari
Hero
A natural 20 still increases the success factor by one step, so it will almost always be a critical hit even if not 10 above the target number. For example, if the AC is 25, you have +8 to hit and roll 20, your result is only 28, but it still critically hits as the natural 20 increases the degree of success from success to critical success.While I like to also use the same rules for crits and such, it should be noted the PF2e has a different system for crits, you need to roll 10 above the target number for something to be a crit so in some cases, a crit simply won't be able to happen.
In gameplay, the "10 above mechanic" mostly means crits happen more frequently, as you can often crit without needing a 20.
But if the AC was 29, you are correct that it would not be possible to critically hit the target with only a +8 to hit. Though I imagine that scenario is one the players are not intended to fight anyway.