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!