All these answers address getting the app window back as focus after uigetfile.
The other side of the problem for me is the uigetfile dialog itself launches behind all other windows.
Here is a workaround I found. This works on 2020a and macOS Mojave
[xl_file,xl_path] = uigetfile('*.*');
app.your_app_here.Visible = 'off';
app.your_app_here.Visible = 'on';
This seems to work both from within the app embedded code and with "external" files
in the main Matlab window.