I have found a bug using the front end submission form - image select from the server type (not the upload ).
The actual select button does not display, I have added some screenshots.
There is a class of "modal" on the href element which of course gives it a "display:none" from bootstrap, fiddling around in developer tools altering the css rules I can show the button and make it work sort of but I would think this needs to be sorted out quickly in the rendering code.
Image 1 shows the "Select" not available
Image 2 shows the class "modal" on the href element and the default rule from bootstrap of display:none
Image 3 shows the altered css
Joomla - 3.8.7
F2C - 6.16.0