I don’t think that’s as uncommon as you might think. As in MF’s case, the appeal found the tribunal had erred in law, and ordered a fresh tribunal
That's not true.
They'd never even got onto the Employment Tribunal bit. Before they could start, the needed to establish whether the belief in two sexes was protected under the Equality Act.
Normally when someone alleges discrimination, it's in a category already understood by the court, e.g. discrimination on the basis of sex, race or religious belief.
No one's ever claimed discrimination based on gender critical beliefs (in biological facts) before, so the court had to be satisfied that it was allowed under the category of belief. The more junior judge found it wasn't and therefore the tribunal couldn't proceed.
The senior judges in the appeal court found that, actually, the first judge was wrong, Maya's belief's (in biological facts) are protected, and therefore the brakes are off and the original employment tribunal can continue.
No one ordered a new tribunal at any point.