Sounds like a puzzler. Do me a favor, if you have another child to add from your GEDCOMpare please attempt to do so.
When the Add Page appears please check for these on that page.
Does it appear at the top- Edit Family of Sándor (Styavniczky) Selmeczy
Then below that does it have- I. Add Child of Sándor (Styavniczky) Selmeczy
Scroll to the bottom of the page and does a check box show up with this after it: Borbála Terézia Rőczey(Rőczey-1) Uncheck this if Borbála is not the other parent. And is there a check in the box?
Also, do the two children already added to Styavniczky-1 appear with their check boxes to remove if wanted to do so? You don't in this case so there should be no checks in their boxes.
If you don't see this, just close that page and nothing will be added to the system and report what you see. If this appears, you should be OK to add the next child.
Since you merged Styavniczky-1 and Selmeczy-7, even if it says it wants to add to Selmeczy-7, Styavniczky-1 should appear as the target Profile. The system knows any reference to Selmeczy-7 should redirect automatically to Styavniczky-1.