This is likely intentional
Can anyone point me to sage advice / RAI clarification / errata?
I was unable to find an official response at the time this question was asked, however Jeremy Crawford recently answered this question directly. See the answer by V2Blast.
I believe you have interpreted the spell correctly, and I believe it is intentionally designed to function in that manner as it follows the same pattern as similar spells.
Compare Zone of Truth:
On a failed save, a creature can’t speak a deliberate lie while in the radius.
To Evard's Black Tentacles:
When a creature enters the affected area for the first time on a turn or starts its turn there, the creature must succeed on a Dexterity saving throw or take 3d6 bludgeoning damage and be restrained by the tentacles until the spell ends.
Each creature that starts its turn in the webs or that enters them during its turn must make a Dexterity saving throw. On a failed save, the creature is restrained as long as it remains in the webs or until it breaks free.
Or Control Water:
When a creature enters the vortex for the first time on a turn or starts its turn there, it must make a Strength saving throw. On a failed save, the creature takes 2d8 bludgeoning damage and is caught in the vortex until the spell ends.
The spells are broadly similar in that:
- Each spell forces a save when a creature enters the affected area, or starts its turn there.
- Each spell specifies an effect the creature will suffer on a failed save.
- Each spell specifies that the condition will last until some other condition is met (spell ends, creature leaves the affected area, creature breaks free, etc...).
Zone of Truth follows the same pattern. It would appear that this is the intended design.