A mystery 300px image size

Updated Answer

After doing more testing, the 300px size only gets generated when I have the official WooCommerce Brands plugin activated.

So 300px is the default WooCommerce image size when the standard sizes go undefined. See wc_get_image_size() in \includes\wc-core-functions.php.

It seems that at some point WC must have filled in the blank of a missing size and that setting got used.

My temporary solution is to deactivate the WooCommerce Brands plugin. Obviously this isn’t viable so I may have to report this bug…

Bug has been reported.

I talked with WooCommerce support and they’ve been able to replicate the issue and have created a bug report.