Update : Following the steps in Össur’s post below fixed Thalyssra for me. If you want to also fix Oculeth and Valtrois you can keep on questing in Party Sync until you bring them to Shal’Aran.
I synced with another character that hasn’t done any Suramar quests yet. When I left the party my Thalyssra was broken and has no interaction options.
It says she’s withering and to give her Ancient Mana, but that option isn’t there, probably because this character is done with the quests here and she’s supposed to be permanently Sated now.
https://i.imgur.com/Aa7UffA.png
Edit : It appears that the Arcane Thirst levels for Thalyssra, Oculeth, and Valtrois are controlled by factions 1860, 1862, and 1919 respectively. For whatever reason, entering the Party Sync set those factions to the Withered rank, but leaving Party Sync did not bring them back to Sated, despite my completion of the Shal’Aran questline.
I’m unable to interact with any of those three characters, preventing me from acquiring their buffs, purchasing items from Thalyssra, or starting the Withered Training scenario.
Edit 2 : Quest #43568, Arcan’dor, Gift of the Ancient Magi, is the quest that sets those three factions to full. It is completed on this character, but somehow the effect of it was undone by the Party Sync.
Edit 3 : I did a Party Sync with a character that has also completed that quest, as suggested by this support article (https://us.support.blizzard.com/en/article/000057727), but that did not fix my issue.