oh, i think i've seen this before!
iirc, "iron bars" are the default object (first one listed in the raws?) that gets used if somehow no other object was provided. (maybe, back in 2017, the first one was coke?)
i believe it's not that the script is "constructing the cabinet on the spot out of iron bars", but rather, something has gotten corrupted, and instead of collecting a cabinet object from the stockpile, the dwarf has "collected" some nonsense unitialised object that's being interpreted as iron bars
i haven't used plannng mode in a while, but i _think_ maybe you can reproduce this on demand by placing some furniture in planning mode, specifying objects you don't have yet (so that it creates a suspended job, which will be unsuspended by the script once the desired object exists), and then going into the jobs screen and unsuspending the job yourself -- some dwarf will execute the job, with whatever placeholder object it had been originally created with...
the simple answer might be "well, don't do that then". but maybe there's a new bug that has surfaced and creates similar weirdness, when the player hasn't manually unsuspended an unready planning-mode job? idk
maybe Stargazer can chime in as to whether they recently unsuspended some jobs just before this happened