[Geoman]

I guess the obvious question would be: Has the engine over-heated recently?
I have had my temperature gauge reading one bar above the middle range this summer and has had no effect on the lettering.

 

[Geoman]

I would check the underside of the engine cover. If it is also melted, that would indicate the heat source from the engine or damaged at the shop. If only the lettering is melted, this would probably indicate that someone in the shop placed something hot on top of the cover. Just a thought...