Hi Mama Mia,
normally this shouldn’t happen. Here are some factors that could cause it:
Ovens can have hot and cold spots and if the cakes are sitting over a cold spot then the centres could take longer to cook, or may not cook properly. If the cake takes too long to cook then the raising agent can also stop working, causing the cake to sink back if it has not fully set.
Also make sure that the oven is properly preheated before the batter is mixed. If the cake batter sits around too long before being baked then again the raising agents will finish working before the cake is baked and the centre will sink back. Also make sure that you are measuring the raising agents carefully (use a proper measuring spoon) as too much can cause the cake to rise very high and very quickly but with a weak structure, so it will sink back again as it comes out of the oven.
Whilst you will need to check that the cake is baked, don’t open the oven door too early. A blast of cold air, generated by the door opening, can also cause the unset centre of a cake to sink. Avoid opening and closing the oven door too sharply and move the pans around gently to minimize the risk of sinking.
While baking cakes, always use fresh and comparatively new raw materials. Take a look at the expiration date of baking soda, baking powder, flour, etc. and use fresh eggs and milk, and most importantly don’t forget to bring the ingredients at room temperature before using. Because sometimes old and damp elements can be a significant reason for the sinking.
It’s essential to allow the eggs and butter to warm to room temperature before you blend them. That is to say; cold eggs don’t mix well with other ingredients and can cause pockets of unmixed batter that may cause your cake to collapse.
I hope it will help!

I noticed your sponge cake has a depression after it is cooled. I get that sometimes. Any advise as how to avoid that.