Gtk+ icon missing when running in MS Windows

This is a late reply but I encountered this issue and I have solved it with the following steps:

SVG icons of the default theme (adwaita) needs to be copyed to a share directory relative to the bin directory bin../share/icons/Adwaita/scalable/* also bin../share/icons/Adwaita/icon-theme.cache and bin../share/icons/Adwaita/index.theme

After several test, it appears that the following files are also required:

lib/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-svg.dll
lib/gdk-pixbuf-2.0/2.10.0/loaders/loaders.cache

The first line of this loaders.cache file also needs to be edited to "../lib/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-svg.dll"

The file tree looks like this:

bin/gtkapplication.exe
bin/*.dll
share/loader.cache
share/glib-2.0/gschemas.compiled
share/icons/Adwaita/scalable/*
share/icons/Adwaita/icon-theme.cache
share/icons/Adwaita/index.theme
lib/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-svg.dll
lib/gdk-pixbuf-2.0/2.10.0/loaders/loaders.cache

Most up-to-date repository containing GTK+3 Runtime files for Windows (as of 5-23-2019): https://github.com/tschoonj/GTK-for-Windows-Runtime-Environment-Installer

As a workaround I have converted the icons from svg to png, then placing them in share/icons. The icons now show up, though there is a minor problem: when the search entry is empty, an additional magnifying glass icon will shop up on the right.

The additional magnifying glass will be replaced by the "clear" icon when there is text in the entry, so I do not consider this that much of a problem... (Although admittedly it looks quite weird...)

As this is only a workaround and not a perfect solution, I will not mark this as accepted answer. I think the problem may be related to how the svg files are displayed, but I have all the DLLs from here so I don't really understand what the problem is.