Not sure how accurate this is, but I found this quote on Bash here
https://fanra.fandom.com/wiki/Skill_...Combat_Effects :
"As a side investigation, we asked the Devs why a bash that failed to stun was so much more likely to interrupt casting than a bash that would have stunned, but didn't (due to resist or immunity). It turns out that the mechanism first determines if a bash will stun or not; if not a stun, it checks for interrupt or not. When bash was put in to the game, they did not take into account stun immunity or the development of stun resist mods; so that if the first check yeilds a (potential) stun, it doesn't check for interrupt (assuming the stun would do the interrupting). The Ogre racial immunity to frontal stuns seems to be well worth the 70 levels of experience penalty at this point."
This logic is basically what I was describing above, except it apparently didn't check for interrupts at all if the stun was resisted. Of course, I am not sure where this quote came from, exactly.