In standard French, the last e is mute so encore is made up of two syllables and is exactly pronounced like encor which would have been indeed a better alternative here, being precisely an invention (license poétique) to align the pronunciation with the classical poetry rules.

I guess either the original text was later “hyper-corrected” or the hémistiche following encore was enough for the author to assert the “e” was kept mute, like it is at the end of a verse.

I’ve always thought that “encor” was used only in 17th or 18th century poetry. But https://fr.wiktionary.org/wiki/encor cites a Boris Vian poem from the 1950’s. So it seems right to replace the “encore” in the Rosemonde Gérard poem by “encor”.