Why is my cake sinking after baking with a soggy-like line in the middle?

Those cakes are underbaked, that line at the bottom is a clear sign that the batter hadn't crystallized when it was removed from the oven, so it could not support the cake above. There's a few things that could be happening:

• You changed the recipe or ingredients: if you've made a substitution or changed the recipe you should look at changing it back
• You aren't baking them long enough: changes in the starting temperature of the batter can change the baking time. In the winter your kitchen may be colder so your batter may be colder, or maybe you're pulling ingredients straight from the fridge, if your batter is colder than you need to lengthen the baking time
• Your oven is cooler: it's possible your oven thermostat is mis-reading and your oven is cooler, or maybe you're cramming the oven with many cakes and your oven can't keep to temperature. Also, maybe you're opening the oven door lots and losing heat. Try using an oven thermometer to check the temperature

I would suggest you start testing your cakes for doneness before taking them out, it's not uncommon for ovens to fluctuate so you can't assume that a cake is going to be done at an exact time. Use the poke test and/or an instant read thermometer to check them.