When using Rellor I used Take Down, but it got repeated and I was not able to select another move resulting in a K.O instead of me being able to catch the Meditite I was battling.
By the way the wild Pokemon was able to use it's turn in between 'my' moves.
Take Down is not a consecutively executed move and Meditite cannot learn Encore in Scarlet and Violet. By far the most likely scenario is that you accidentally clicked Rollout instead of Take Down, which Rellor learns at Level 11 and would plausibly still know while you were using it.
Yeah, that was pretty wierd.
It was definitely a glitch. No Pokemon (Except Kanghaskan... kinda) should repeat a move that isn't multi-turn. Additionally, Meditite has no ability that would trigger this, nor Rellor.
Such a mechanic doesn't exist.