Field content in form sometimes changes after running a query

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I have a form called Products. Besides lots of other fields used to capture product specific data, there is a field called Category from which the data is created by means of a populated combo box. The values for the combo box are from a product category list which I typed in.
I have another form which is created to run some preset queries from which the users will run queries by means of a button click
One of the queries is a search by product Category . Another combo box is used with the same values list that is on the Product from. Users select the product category they wish to search on and by clicking the appropriate button the query is run and displayed.

This generally works fine but sometimes I find that after running a category query the category field content for record 1 on the priducts form gets changed to a different category.
I'm trying to find what sequence of events causes this to happen, so far without success.
I think it's something to do with leaving a query open and then running another Category Query from the Query Form
Does anyone have an idea as to what causes this to happen - It's got me beat at present
thanks
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