When you create a View you are asked to select the Post Type that will be handled / queried / returned yb this View.
Filtering by the Post type is not possible natively exactly due to this.
It's not something we intend to provide, as usually, performance concerns will determine that the user selects as few as possible Post types to query.
For a Fully working post type filter as a front end parametric search you would need to select ALL post Types, producing a HUGE and expensive Query with this View.
Anyway, the issue you experience I think I am handling it here:
And I believe it is a BUG.
See, if you set this all up with native Views Features, it still removes the URL Param and just queries by the Parametric Search:
1. Set a view to return Posts by any term or meta, set in the URL parameter
2. Call this View, passing the URL parameter ==> All results are correct
3. If you now in the View have a parametric Search by something else, and use it, it simply removes the passed URL parameter and replaces with the Parametric Search.
This does NOT happen, if the URL parameter is passed by the parametric Search itself.
It ONLY happens, if the filter is NOT preset in the Parametric Search HTML Section.
If you can confirm this I will escalate it as a BUG
I apologize to inform that this will not be solved in the upcoming Views Release.
But we will handle this straight after the next release which will be very soon.
Thank you for your patience and understanding.