This is pretty interesting.
I wouldn't call this a bug but more of a feature request.
If the destination drive is full, the file placement balancing pass needs to enumerate the destination drive and see if it can find files to move out of that drive without breaking any existing rules. This would add additional overhead, but it sounds like a reasonable request.
This should probably be an option for each rule that the user can enable on a rule by rule basis.
I'll start thinking about how we can implement this.
----
For now as a workaround (and I realize this is not ideal), you can use the Drive Usage Limiter to evacuate all the other files from that drive to make more room. Just make sure that Balancing plug-ins respect file placement rules is checked and Unless the drive is being emptied is not checked.