Strange results from assume (maybe a bug?) ad asksign

Sorry, in my previous email I was wrong about something, please do not
consider "asksign" from now, the problem is "assume" . Anyway I'm
confused... but I think I found a bug. Let's try this:

1- with numerical values:

(%i1) assume(a>1,a<2);
(%o1)                           [a > 1, a < 2]
(%i2) is(a<0);
(%o2)                                false
(%i3) is(a>2);
(%o3)                                false
(%i4) is(a>1);
(%o4)                                true

that's ok. In my previous email I thinked %o4 should result in an
"unknown" but I was wrong.
But let's see what happens with special numerical values like %pi:

(%i1) assume(b>0,b<%pi);
(%o1)                          [b > 0, b < %pi]
(%i2) is(b<0);
(%o2)                                false
(%i3) is(b<%pi);
(%o3)                                true
(%i4) is(b>%pi);
(%o4)                                false
(%i5) is(b>2*%pi);
(%o5)                                false

That's ok. But attention to the last output, with is(b>2*%pi). Now
comes the problem!!
Try this:

(%i6) assume(c>0,c<%pi/2);
                                           %pi
(%o6)                          [c > 0, --- > c]
                                              2
(%i7) is(c>0);
(%o7)                                true
(%i8) is(c<%pi/2);
(%o8)                                true
(%i9) is(c>%pi);
(%o9)                               unknown
(%i10) is(c>2*%pi);
(%o10)                              unknown

I think that defining a bounded variable with numbers is ok, also with
%pi, but not with fractions of %pi, nor with fractions of %e !!! It
seems that Maxima, when in the assume condition there is a fraction of
a special constant, redefines that constant in function of the
variable that I try to bound with assume. It is evident also by the
output:

(%i1) assume(b>0,b<%pi);
(%o1)                          [b > 0, b < %pi]

(%i6) assume(c>0,c<%pi/2);
                                           %pi
(%o6)                          [c > 0, --- > c]
                                              2
In %o6 is %pi/2 that was redefined to be > c, in %1 is b that is
function of %pi. It seems like the roles was turned upside-down...

Is this a bug?
Or there is a way, like an option variable, to avoid this? Or what is
the correct way to bound a variable within two limits? I need this...
Thanks, Stefano