Comment 1 for bug 1572812

Happened between 6608 and 6663 (lowest in the nightly list, I don't have anything prior) by some deductive testing (builds > 6663 have this behavior present).

Build 6608, tested by Joan_Sparky, had no issue:
https://forum.kicad.info/t/schematic-symbol-renumber-doesnt-hold-value/2854/5

Production build 4.0.2 also behaves normally.