RAW is ambiguous, but as a DM I would be inclined to say if it's turned it's turned - and yes, as stated, an undead that has legendary actions is likely to have legendary resistances AND turn resistance. If somebody got through all that and managed to successfully turn the creature, I think the DM should let them have it.
In terms of it bringing an anticlimactic end to, say, a campaign-ending fight, it doesn't seem likely since in most cases by virtue of the plot the adventurers will want that creature destroyed, not just "running away for one minute". And damaging the creature breaks the turning effect. So turning a really powerful undead during a climactic encounter could provide a huge advantage but probably not a decisive end to the creature. And if somehow the undead does end up fleeing the encounter completely, you have a recurring villain for a future campaign.