Something else I thought of: Soleta was raised in Vulcan society but was never mentioned to be betrothed. So maybe she was determined from an early age to not experience pon farr? Plus, the mirror Soleta was raised in Romulan society and no one ever figured out her half-Vulcan nature for decades.
Well, it makes sense that no one would realize that Soleta is "half-Vulcan," since Vulcans and Romulans cannot plausibly be separate species yet. Less than 2,000 years of separation is just not enough time to speciate.
As for why Soleta was not betrothed -- perhaps she comes from a Vulcan culture that doesn't practice betrothal? Honestly, I'm kind of surprised that a Federation that values individual liberty would tolerate forced marriages.
Not even all full-blooded Vulcans go through pon farr. Peter David's New Frontier: Excalibur: Renaissance
states that homosexual Vulcans to not experience pon farr, and as a restult, do not procreate.
Fascinating! One would imagine that Vulcan has its sexual minorities, too.