Hello,
I like to repost this problem because there was a mistake when speaking about the page where the issue happens.
In e-com.express theme I have set the default image by drag and drop editor. However, when a visitor clicks the default product image he/she goes to the product page where the default image is not visible as the first image. There is a variant image instead.
Tried to fix this in the Sections and editing product-template.liquid and changing the first line:
{% assign current_variant = product.selected_or_first_available_variant %}
to this:
{% assign featured_media = product.featured_media %}
This fix worked and changed the default images to be the first ones at every product page.
Unfortunately, after this all the product pages lost Buy Now and Add To Cart buttons.
A piece of additional code somewhere?