weird capitalization

Using maxima 5.6, i found that some letters are converted into capital
case. It happens, e.g, for g1', while not for g, g2, g3:

(C3) a : g ;

(D3)                                   g
(C4) a : g1;

(D4)                                  G1

(C6) a : g2;

(D6)                                  g2
(C7) a : g3;

(D7)                                  g3
(C8) 

Probably it is nothing to worry about, but how to fix that?

thank you 

--
Paolo Pumilia