Sorting by field included in only one content type, excludes all other content types

My search-api server (db search) indexes three different content types, let's say
"content-type-A", "content-type-B", "content-type-C".

The first content type, "content-type-A" has a "referring date" field (a simple date
field); the others two content types haven't this field at all.

I built a views to search in the index, and it works perfectly. But when i
add a "sort" option to sort by that "referring date" field, the view just
lost several results: only the "content-type-A" results are given; i've
several pages of results in the "pager" but only the first pages are working: if
I click on the last pages, they're just empty. Seems that the view just
exludes contenty types B and C, since they don't have that date field.

I expected that all results are shown, maybe putting content types B and C results
in latest positions... but the view just "ignores" some result. And the pager also
dont work as expected, since i have just "empty" pages.

If i remove the "sort" option - it works again, I have the full results set.

What should i do to "custom" that search so that when a "referring date" field
is not present in the content type, the content type's results are shown anyway
and not "ignored" by the view?

Really thanks