Thanks for reporting this. It seems that with the most recent WordPress update (5.2) the flag HTML was being stripped out of the block editor.
A new SVG flag shortcode has now been added to the plugin! (v0.4)
Just add: [svg-flags flag=”gb”] to display an inline SVG flag. See plugin settings page for full usage.
You can still use the SVG flag HTML if you add it to the HTML block in the new editor. But if you just add it to an existing paragraph block (for example) the editor strips out the flag HTML.
But with the new SVG flag shortcode you can add an inline SVG flag to any block. The only downside to this is that you won’t see the flag displayed in the editor, it will only be displayed on the front end.
I’m also considering adding full support for a dedicated SVG flag block that will support a full visual interface in the WordPress editor. Let me know if this is something you might be interested in.