Overall priority not decreased transitively

TaskCoach 0.71.5 on Ubuntu 8.04

In a tree of tasks A, B, C with A containing B and B containing C, decreasing the priority of C will correctly decrease the overall priority of B while not making it less than the priority of B. This is expected behaviour. Simultaneously, the overall priority of A is "transitively" decreased, too, as expected, but the limit here is none of the individual priorities of A, B or C, but rather some priority value of unknown origin.

Effectively, this leads to a very annoying situation where I can't get task A off the top of the list of tasks if I sort it by "overall priority", because TaskCoach doesn't allow me to reduce A's overall priority beyond that fixed boundary. (Increasing the priority, by contrast, always works well.)

In my case, the problem disappeared (i.e., the overall priority of A was re-set to the maximum priority out of those from A, B and C) when I did "Purge deleted items", then restarted TaskCoach. Unfortunately, this means that I can't furnish any minimal sample.