loss of formatting information

[KHZ (SAW)]

Hi world.
 
My experience is that if  single cells are formatted they exist in Excel
(contrary to format whole lines where formatting is lost for newly
created cells).
 
If such a cell is filled it doesn't loose the formatting. Now I've a
case where cell A1 is prefixed by ##1- and the format information is
lost.
 
Any idea?
 
Regards,           Karl-Heinz.