Have a look at this answer:
https://wordpress.stackexchange.com/a/243284/64435
If you do not create multiple versions of same proportions no data-src-set will be loaded since you have only one possible target
Have a look at this answer:
https://wordpress.stackexchange.com/a/243284/64435
If you do not create multiple versions of same proportions no data-src-set will be loaded since you have only one possible target