I used full fat yoghurt for the Gluten-Free Banana Bread, so the batter was very wet. It completely sank in the middle after baking. I don’t know why.
Nigella's Gluten-Free Banana Bread (from COOK, EAT, REPEAT) uses Greek yogurt, which helps to give the banana bread a moist texture. Greek yogurt is strained, so it thicker and has a higher fat content than regular whole milk yogurt. If regular yogurt was used then it would have slightly less fat and slightly more water and this could make the batter a little more runny and slightly affect the baking time.
Gluten-free cakes often sink slightly on cooling. The banana bread will not rise much during baking and may sink slightly in the centre on cooling, but should not collapse competely. If it did collapse then it is likely that the banana bread had not quite baked fully (and in light of the ingredient change may have needed a slightly longer baking time). All ovens vary, so the baking time is a guideline. When the banana bread is ready it should be firm on top, starting to shrink away from the sides of the tin and a cake tester inserted into the centre should come out clean or just with the odd crumb attached to it, but not unbaked batter.