ASCII characters with the high bit set are extended ASCII codes. As far as I know, all
compilers that accept ASCII codes between the double quotes only work with standard
ASCII codes. Not extended codes.

I have tried your example with extended ASCII codes "°öµC" with versions 2.47 and 2.5
of PBP, another BASIC compiler, and two C compilers. They all output the same garbage
where each extended ASCII character is between the quotes. All non extended codes
print as expected.

PBP 2.50 seems to be the only compiler that shows the warning, but they all print junk
on my LCD where extended ASCII codes are located in the string, between the "".

Did you get PBP v2.47 to actually print these extended ASCII characters "as shown", in
between the " " on your LCD, or did it just not throw the warning?

I'm just curious because I get the same results with 5 different compilers.