"See all" link inside News modern web part is not respecting the Filters we define inside the webpar

Maybe you can create a subsite for that category and show-all only shows news posts of that subsite.

Update (19.09.2019):
The filters started working now.

You can set the filter based on particular properties and the filtered content will be based on that property.

To set property, open site contents and select the page/file you wish to set property elements and add a column to that or edit existing.

Then use that specific property in the filter section.