Appling a debuff goes thru 3 possible checks (technically 4) if the one before fails it doesn't matter about the others if any fails than it doesn't apply
1. Evasion - > is the abily evaded (dodged/deflection). Some abilites can't be evaded
2. Process/application rate - > this is in the ability description. If no % chance to apply is given than process rate is 100%. Some require other things to process like a crit hit
3. resisting - > tenacity vs potecny check. Chance to resist is tenacity - potency where chance to resist can't go below 15%, this is what the Dev's have stated it is. Some abilites the debuffs can't be resisted as stated in the ability descriptions and tenacity down can never be resisted.
The 4th check is actually 1st and that is immunity. Some characters (jyn with unique zeta is immune to stun) are immune to certain debuffs so it just skips all the checks and doesn't even attempt.
To the claim of every Debuffer gets 100% potency if it does state a % chance to apply is false. The Debuffer has 100% chance to "process" so it skips step 2 above but still goes thru step 1 and 3.
Sion AoE skips steps 1 and 2 since it is unavoidable and doesn't state an application rate however it still goes thru step 3 based on his modded potency plus any lead, buff or unique potency he may get. If he got 100% potency because his AoE doesn't give a application rate than zeta bastilla Jedi would rarely resist his debuff because he would have close to 200% potency with a traya lead and that.
So not having an % to apply just means 100% chance to apply (process step 2) still goes thru resist check (step 3)