Listgrid header problems in a tabset

Hi,

I have a tabset with two tabs. Both tabs contains a listgrid where one of the columns is getting the data from a database table, so in the listgrid header should appear a combo to filter values.

The combo is appearing in the listgrid of the second tab but not in the grid corresponding for the first tab. I checked the problem appears in the tab that is set as default

The problem appears in Chrome and IE 9.

Any idea to fix this issue? Thanks