Skip Images Classes, not workin

Hello,

I do not understand how the “skip images classes” option works, because for me it doesnt work.
For my HP, if i want to not have lazy loading on : class = mvp-feat3-main-img

I tried as it says in the plugin :
mvp-feat3-main-img
and I even tried .mvp-feat3-main-img

The lazy load works, but the exclusion does not.
Any idea of what is the issue?
Thanks !

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