Since we haven’t had any clarification on this despite it being asked on multiple threads, I thought I’d share my very recent (past 20minutes) experience with this.
I used 2 cans and natural energy for 18 raids. Hengyen got to hit 3-4 times a raid so it is a sample size of 60-70. Not large enough for a complete guarentee, but reasonable enough to suggest the conclusions are correct.
Every single first hit managed to crit. Every second normal hit on the AR managed to crit (far smaller sample size of 18 for this one). However when he got a second bonus hit due to his weapon, it didn’t crit.
So my conclusion would be that it is a 100% chance of a crit, but this doesn’t affect the bonus hit due to his weapon. This second part obviously isn’t an issue to the vast majority of Fast characters.
Hope this information helps.