After a bit of searching in Meta Stack Exchange, I found this answer in which it was mentioned:
If the review task is removed due to other circumstances outside the queue (e.g. a moderator deletes a post that is being reviewed, outside the queue), it is marked "invalidated" or "no longer reviewable".
But in this case, a moderator did not delete the post.
Even this bug is not relevant as the OP did not delete and undelete their post.
So, why was the review invalidated?