Is pausing at certain words in Surat Al-Fatiha during prayer, unintentionally, considered an unseemly pause, and what is its effect on the validity of the prayer?

"Ugly وقف (Waqf Qabih)" is the act of stopping where the meaning is incomplete and not disconnected from what follows. Stopping at the words mentioned in the question is considered an "ugly وقف" and should not be intentionally done. The reciter who stops at such a point must re-recite from before the point of stop. This ugliness is in the terminology of the reciters (Qurra'), not a sin according to Islamic law. There is no obligatory or forbidden وقف in the Quran except for what has a specific reason, and there is no وقف for which a person is sinful by performing or omitting it according to Islamic law.