I have the same issue.
The filename-field is empty, and its almost impossible to write a filename into it.
When you click on the field and press a key, this key is put into this filename-field, but the later keys are put into the search-field.
I have managed to save one file, with the filename "a" without any more, had to change the file to a .gcode afterwards.
The typing into the search-field brings the software to an unusable mode, i had to cancel cura the hard way.
I am using Cura with an Arch-Linux, Mate-Desktop.
Edit: Seems to be a problem of a library creating this Fileselect-Box. Happens to me in Brave-Browser, too, when trying to save an image.