iFIX 5.1 Text animation source does not change

Hello! I have a problem with iFIX 5.1. I have a group of "Text" objects. Some of them have several animations, some don't have any animation. Here is a sequence:
1. I open group in System Tree,
2. then open one of "Text" objects,
3. mouse right button click on "BackGroundColorAnimation", select "Properties".
4. The Property "Source" is currently "Fix32.THISNODE.STATE_TANK_6.F_8 + 2 * Fix32.THISNODE.STATE_TANK_6.F_9 + 4 * Fix32.THISNODE.STATE_TANK_6.F_10"
5. I change "TANK_6" to "TANK_3" everywhere, then press "Enter".
6. But property value does not change. It returns to previous value.
7. If I change it several times in a row then it can return to any of previous values that I entered before!

So, I actually cannot change "Source" value. VBA scripts also can't change "Source", and "Ctlr+F->Replace" can't too.

How can I fix this? :confused:

P.S.: If I create new Text object and add animation to it, then it works fine.