WP Bakery Page Builder custom sizes not compressed

Hello I’m currently testing SPIO in order to implement it on sites using WP Bakery Page Builder.
Actually, this plugin propose elements where you can define a custom image thumbnail size (other than the thumbnails sizes available for the whole site, which show up in the SPIO settings). Once defined, and the page updated/published a new image file gets generated in the corresponding uploads folder (i.e. myimage-151×301.png if I entered “151×301”).
The problem is that such image files don’t get compress by SPIO :/
Checked:
– The Statistics tab don’t show new spent credits.
– If I check 2 comparables image files in the uploads folder I get such sizes:
> myimage-150×300.png (sitewide thumbnail size, shown in SPIO settings) = 8ko
> myimage-151×301.png (custom thumbnail size defined within a WP Bakery element, not shown in SPIO settings) = 24ko
More info:
– “Optimize other thumbs” is checked in the “Advanced” tab
– I’m using Route theme which requires/recommend “Regenerate Thumbnails” plugin (which SPIO propose to deactivate, once SPIO is activated). Tests have been made with “Regenerate Thumbnails” plugin activated and deactivated, and also with “Regenerate Thumbnails Advanced” instead: the one developed by the Short Pixel team (not sure if my settings where good in that case…)

Hope you can help, SPIO seems really great so far.
Any clue? Advice? Workaround?
Thanks in advance to who might help!