It isn't a bug, so much as an oversight. That trait lowers the base maintenance, not the maintenance paid.
You can fix it in mods though. Make the base maint. paid be 100%. Give all ship sizes 75% maintenance reduction, and bases 96% (IIRC). Then, 1 point in Maint Redux actually lowers maintenance paid by 1%.
Of course, then you need to raise max level of Maint Redux and lower its cost and raise the threshhold level, as 10% becomes worth less than 500 points. Look at the Pirates and Nomads mod for an example of a mod that fixes the Maint Redux trait.