Interesting! First, we
can ignore compressibility and therefore EAS. It is true that IAS at the stall
increases with increasing height but this only becomes significant at high
altitude and higher Mach numbers.
At heights up to 10,000ft
you would expect your stall IAS to be constant - for all practical purposes -
although air density is significantly lower.
Simple maths on the lift equation will show that if rho goes up V
goes down. But V is not IAS, it is TAS. What is happening is that the same
density change that shows a change in V is also changing the IAS/TAS ratio so
IAS remains the same. It is your TAS at the stall that has decreased.
Returning to EAS, at
height rho is corrupted by compressibility and while stall EAS remains constant
stall IAS rises. Even higher CLmax is contaminated by shockwaves and even EAS
at the stall rises