First up, this would be a NWN bug, so there isn't anything we can do.
I can reproduce this when I run past hostile creatures who aren't "in" combat with me (yet), causing Tumble checks to be made, even though I've got Spring Attack. However, once the hostile creatures are "in" combat with me, I can't cause any further Tumble checks by moving around.
Bear in mind this was tested on a single player local module, whereas on a remote server, the effects of the network may influence the outcome.
Hope this helps.
Regards,
Script Wrecked.