Looking at the codebase, it seems as though the whiskers on the MRIQC boxplots signify “the closest value to [1.5x] the IQR without going past it” (that link goes to a code comment from the MRIQC repository where the whisker value is calculated). So, the whisker is never going to be exactly 1.5 unless you’re very lucky with your values, but should be close.
Let me know if that makes sense in terms of what you’ve calculated (to confirm I’m not totally misreading the code )!