SO line: problem in From stock/From order value

I'm working on the latest trunk version.

When I create a new SO, and then add SO lines (for ex. PC1 which is defined on product form as "From Stock"). Then, on the SO line, I decide to change it to "From Order" for this SO. Then I change the qty and then validate it. The problem is that at that time, the system will re-change the "From Order" into "From Stock" again (If I re-open the SO line that I validated, I see that it's "From Stock" now). That means that if I want it to keep my change "From Order", I have to do it last, after any other change (like qty, price, etc), otherwise the system will not keep my change.