Are the Pali-words Patinissagga and Vossagga synonyms? Are both Pali-words synonyms or is there a slight difference between 'patinissagga' and 'vossagga'?
While not always, paṭinissagga is often used for more lofty situations, such as connected to Nibbana (in SN 56.11; AN 3.32 &amp; MN 118).
Vossagga appears always used in less lofty situations and appears more 'personal'.

vo
enclitic from tumha
to you; of you; by you

Each use of each word in the suttas should be examined; as for the etymology of each word. A post on the topic is here.

MN 118 contains both words. 'Vossagga' is found describing the maturation of the qualities each factor of enlightenment relies upon. Therefore, 'vossagga' appears used with path factors Anapanasati relies on.
'Paṭinissagga' is used to describe the end of the path of Anapanasati. Therefore, 'paṭinissagga' appears more lofty than 'vossagga'.
Start with 'vossagga' and end with 'paṭinissagga'.

It’s when a mendicant develops the awakening factors of mindfulness, investigation of principles, energy, rapture, tranquility, immersion, and equanimity, which rely on seclusion, fading awa, and cessation that ripen as letting go.
Idha, bhikkhave, bhikkhu satisambojjhaṅgaṃ bhāveti vivekanissitaṃ virāganissitaṃ nirodhanissitaṃ vossaggapariṇāmiṃ. Dhammavicayasambojjhaṅgaṃ bhāveti … pe … vīriyasambojjhaṅgaṃ bhāveti … pītisambojjhaṅgaṃ bhāveti … passaddhisambojjhaṅgaṃ bhāveti … samādhisambojjhaṅgaṃ bhāveti … upekkhāsambojjhaṅgaṃ bhāveti vivekanissitaṃ virāganissitaṃ nirodhanissitaṃ vossaggapariṇāmiṃ.
They practice breathing in observing impermanence. They practice breathing out observing impermanence.
‘Aniccānupassī assasissāmī’ti sikkhati, ‘aniccānupassī passasissāmī’ti sikkhati;
They practice breathing in observing fading away. They practice breathing out observing fading away.
‘virāgānupassī assasissāmī’ti sikkhati, ‘virāgānupassī passasissāmī’ti sikkhati;
They practice breathing in observing cessation. They practice breathing out observing cessation.
‘nirodhānupassī assasissāmī’ti sikkhati, ‘nirodhānupassī  passasissāmī’ti sikkhati;
They practice breathing in observing letting go. They practice breathing out observing letting go.
‘paṭinissaggānupassī assasissāmī’ti sikkhati, ‘paṭinissaggānupassī passasissāmī’ti sikkhati.
MN 118

In conclusion, 'vossagga' may possibly mean when the 'self' or 'will' lets go by employing mindfulness to bring to mind right view. Where as 'paṭinissaggā' may possibly mean when the mind itself lets go due to direct seeing or vipassana.