This is either by design (WHY?) or a bug.
Perhaps this is not a bug, but simply an under-documented property.
The fact is that when a mouse drags a text block, the formulas in the TxtHeight, TxtWidth cells do not take an absolute value, but are converting to the form Width*a, Height*b. That is, the coordinates of the mouse on the go are converted into a multiplier value. Therefore, to avoid the effect of racing or complex calculations, developers could impose a restriction on the formulas TxtLocPinX, TxtLocPinY, so that they too could have a similar form (namely, TxtWidth*a, TxtHeight*b).
This requirement seems reasonable. The only bad thing is that the developers forgot to tell everyone else about it.
Visio 2007 behaves similarly.