After researching “Folding Wingmirrors”, and upgrading the “Fit Wing Mirrors” slot, all but two stockpile slots at the station still contain regular "WingMirror"s. The other two slots are left for "Folding WingMirror"s and have to be re-filled after every vehicle. “Predictive Stock Control” is upgraded at the “Fit Wing Mirrors” slot. All vehicles being built have “Folding WingMirrors” as an option.
Workaround
Changing the “Imports” from both being “Prefer local” (regular “WingMirror” was set to “Prefer local” prior to the upgrade) to both being “Any” refunds all the regular “WingMirrors” and changes the stockpile to require 16 "Folding WingMirror"s as expected. Changing both Imports back to “Prefer local” afterwards does not make the problem reappear.