Is it true that in some cases, deleteing last portal row deletes from another?

Nope, not that I have ever seen or heard. (but I don't know everything either...)

Related records might be deleted if the delete option is specified in a relationship between an occurrence of the portal's table and an occurrence of another table (can be in a completely different part of the relationship graph) and two portals can be based on the same table.

Both might make it look like unrelated data was being deleted.

I believe you mean this, JG:

"FileMaker 16.0.1 data integrity disclaimer"

• http://help.filemaker.com/app/answers/detail/a_id/16465/kw/delete%20portal%20row

Beverly