Unable to filter rows in a sheet populated by QUERY

I am trying to filter one column A-Z to pair items listed in more than one selling app.
However, if I apply an A-Z filter for a column with item codes, the sheet rearranges everything per the filter for about one second, then it inserts a few blank rows below the headers and undoes the filter I applied.

The query looks like this:

=QUERY(
        {Poshmark!A2:M;Letgo!A2:M;Offerup!A2:M;eBay!A2:M},
        "select * where Col1 is not null",
        0)

In short, I see the filtering for a split second, then it undoes itself and a few blank rows appear between the headers and the content.

Any ideas on what to do to be able to freely apply filters?

Thank you!