LEFT JOIN and JOIN behaves differently since upgrade to 10.4

I had to upgrade to 10.4 yesterday from 10.1. I went through each major version and ran mysql_upgrade after each.

Today I noticed strange problems on our selects:

This select returns 1 result:

SELECT prod.* FROM product_groups_790 AS prod JOIN product_site_publication_index_790 AS pr_publ_index ON pr_publ_index.product_id=prod.cod_varianta AND pr_publ_index.is_published AND pr_publ_index.site_id = '83' WHERE prod.cod_varianta=6922 

This select returns 0 results:

SELECT prod.* FROM product_groups_790 AS prod JOIN product_site_publication_index_790 AS pr_publ_index ON pr_publ_index.product_id=prod.cod_varianta AND pr_publ_index.is_published AND pr_publ_index.site_id = '83'

The same selct on the same database structure on version 10.2.25 returns a a whole lot of results.

I noticed similar problems on LEFT JOINS, even if there is data to be joined, it is NOT joined and the result set is empty, unless there is a condition on the main table on the left side.
The left side select should not affect the joined result.

I am not sure which version is related to (10.3 or 10.4) nor where to start to debug.

I tried setting optimizer switch for 10.1 defaults but no change:
https://mariadb.com/kb/en/library/optimizer-switch/