I've also encountered this problem (for Karna, Slade and Gerhalt so far). After some experimenting I'm now able to reveal this mystery:
First, you have to know, how the determination of the turn oder in SF works.
The turn order is determined at the beginning of each turn. The agility
value of each battle participant is taken and altered by the RNG. The variation is +/- 12,5 % of the base agility
value (including equipment and boost) and an additional +/-1. Now the one with the highest agility
value starts this turn and is followed by the one with the second highest and so on.
This system works without problems as long as the altered agility
value is <128 or
the original value (not counting boost or equipment!) is >=128!! Because if none of these two conditions is fulfilled, the character skips a turn!
There is one cap, that prevents you from going over AGI = 100 by using Quick Chickens or by level ups. So it's impossible (without editing) to go beyond this border. With editing you can
go beyond 128, which results in getting double turns like bosses.
But now "Boost" enters this calculation. Boost increases your agility
by 37,5 %. So if your agility
value is high, can get near 128 by boosting. There is another cap, that prevents you from getting over 127 by equipment and boosting. So by now everything should still be fine. But know comes the oversight: This last cap works before
the variation! That means with the help of a positive
variation, the altered agility
value can go over 127 so the boosted character skips a turn!!
Of course the closer the boosted agility
value is to 127, the higher is the chance of being skipped, the highest possible chance should be about 49%.
Here is an example calculation for better understanding:
Base Agi = 90
No Quick Ring or Thieve's Dagger equipped
Character was just boosted
Now his new agility
value is 90 * 1,375 = 123 (all results are rounded down)
After variation the altered agility
value is between 123 *0,875 - 1 = 106 and 123 * 1,125 + 1 = 139.
So there's a realistic chance, this character has to skip
this turn (maybe about 30%), when the RNG decides to go beyond 127.
The other way round: The lowest base agility
, where you have a chance of being skipped is 83.
83 * 1,375 = 114
114 * 1,125 + 1 = 129 > 127
Hope I was able to describe the problem in an understandable way!