Here's the response I got from WooCommerce extension support with regards to my selected featured image no longer being available once I select a swatch image:
"This is not an issue and is the way the extension is supposed to work as usually with products with swatched you do not use a featured image only swatch images. .....If you use a featured image you have of the tacklebag filled with products is not related to a product variation so it will show when the page first loads, if you click a variation swatch that variation image will then load removing the featured image. As the featured image has no swatch you cannot select that image to display again hence the reason you cannot see it again until a page reload."
I hope this explains things a little better.