I used to use 91 to clean flux and it works fine, might have to do a few passes though
If you clean flux regularly I highly suggest getting a cheap ultrasonic bath and some branson EC solution. It comes concentrated so a bottle lasts forever and solution can be reused a few times. If you only do a few projects a year it’s way overkill but I do a lot of rework stuff and it’s soooo much easier to just drop stuff in the bath for 20m when I’m done. A used decent sized one was like $80 and the solution was like $40 for a quart which has lasted me 2 years so far because I only change the bath once a month and it’s like 2% per gallon. Also super handy for cleaning corroded pcbs, stuff like that if you do repair work