Just a few WAG's here. Bootstrap 4 incorporates a lot that responsive_classic does not. Ajax Search, some image handling and control of columns in Product Listing.

I'm thinking you may be loading extra things with bootsstrap that are fighting with these features.

I would first check Admin >> Configuration >> Product Listing >> Product Listing - Columns Per Row to make sure it is set for 0 versus the recommended 3. This will allow bootstrap to properly work with your product images.