What you are seeing is a common side effect of trying to synchronize
comboboxes on continuous form. When you use this technique on a continuous
form you get strange looking results because though you see multiple rows,
there is actually only a single combo box control with a single rowsource.
Whatever you do to that control will be apparent in all rows. When you
restrict the values in the combo box by some other value (Category in your
case) and you have a visible record that does not meet that criteria the
combo box for that row will appear empty. This is a little disconcerting to
the end user so you have to do a little extra work to avoid this problem.

To create the effect of a synchronized combo in a continuous form create
another textbox control, include the display column of the combo in the
Recordsource query of the form (join the foreign table and drag in the
column). Position and size the new textbox so that you can place it
directly over the combo box allowing only the down-arrow portion of the
combo to show.

Create a GotFocus event for the textbox that only does a SetFocus to the
combo.

This will create the effect that you are wanting - the correct display value
will always show even though the value itself might not be fit the criteria
that is in effect based on the current record.

Here's a link to my sample database that offers several different
alternatives for displaying this type of data including the technique
described above:
http://www.daiglenet.com/msaccess.htm

Also, here's a KB Article that explains the problem
ACC2000: Combo Box in Continuous Form Shows Incorrect Data
http://support.microsoft.com/default.aspx?scid=kb;en-us;208866