P: Keyword Set panel re-opens every time Lightroom is launched

Hello to the Community!

This is my first post and, unfortunately, is for a possible bug, not a nasty one but it annoys me a bit:

The "Keyword Set" sub-panel under the "Keywording" panel stays expanded no matter how much I close it. Upon Lightroom re-start I find it opened. It occurs with Lightroom 3.3/3.4 running on Snow Leopard 10.6.x.

Does this happen to any others?

Regards,
Stefano