Thanks Parici

Then it beats me! you have a curiosity.
I'll dig out my 2 Hungarian Catalogues, but I envisage a language barrier.
I can see no reference to this.
Philatalia Hungarica 1956 adds these notes to the "Great Military Leaders"
" Colour shades, blotches, and parts missing in tinting, Flaws and parts missing in surcharges"
So I guess poor quality control at that time, a problem with your scanner,
or light damage, may be responsible.
You'll need a specialist
Magyarorszag Belyegei quotes Zumstein and Yvert numbers
of 722 and 628 for this stamp, if you have those catalogues.