I think it might be the tie. In Dorico 4 & 5 if I enter a syllable on a tied note and then pressed the right arrow I always get the lyric extension line. Try undoing the tie, then entering the word then put the tie back. If I’m doing vocals I don’t enter ties during note entry. When I enter the lyrics I use the right arrow as you mentioned and don’t get extensions. Then I go back and add all the ties.