I found a simpler way to trigger an effect that looks very much like the same thing: hit ctrl-0 (the shortcut for resetting font size) twice after having "zoomed in or ctrl-+'ed a few times and the font size will decrease twice. Another symptom: when I try to return to larger font size by hitting ctrl-+ after this ocured, the first increase in font-size is unproportionally large - as if it returned to the previous size rather than calculate a "relatively proportional" increase.

I cannot reproduce this anymore with Qt5/current master. The fontsize/zoom-level is reset to default/100% on preferences->apply/save and hitting ctrl-0 even many times only changes zoom-level once...