Cheesecake is a custard, so your very best indicator of doneness is temperature. There are a range of opinions, from about 150 F to 165 F / 65 C to 73 C, measured at the center of the pan, in the center of the layer.
If you don't have an instant read thermometer, generally, you want a thin knife to come out clean from the center; similarly, when the pan is shaken a bit, only the center should jiggle slightly. These indicators are not as good as temperature, so you will need to gain experiene to interpret them.
Once you are measuring properly, problems with the top of the cheesecake being done, but the inside being too unset may be mitigated by:
- Baking in a water bath (which you indicate you are already doing)
- Baking at a lower temperature (within reason; you indicate you are already baking at a fairly low temperature)
- Baking in a wider pan, so the cheesecake is more shallow
- Do not use convection
Since you area already using a bath and a low baking temperature, if you are still having problems once you start checking for doneness by temperature, I would suggest checking your oven's calibration with an oven thermometer to be sure it is not running hot.
This is an issue I've had to come to terms with myself. I spent most of my catering life spoilt by having a massive fan assisted electric ovens with space for 24 trays at once. Then one day I left it all behind to work in a tiny 2 chef kitchen where all we had was a bottom heated gas oven. The first 6 months was a nightmare. It's still not easy even to this day but I'll share a couple of tricks me and my colleague have found.
Its all about the airflow:
Forget about the middle shelf for baking it's useless. It's there for roast joints and ... Stuff. Get your baked goods on the top shelf. The reason you are not getting browned tops is all the heat is hitting the bottom of your tray, by the time it reaches the top of your oven and bounces back down to your food it's nowhere near the temperature required.
In order to help cushion the bottoms of your food and direct the heat towards the top, you need to put a tray slightly larger than the tray you are cooking on, on the shelf below. You can add water to this tray for bread and Yorkshire puddings as the steam helps regulate the heat also, but when cooking pastries I find it makes the pastry more likely to split and crack.
Locate the thermostat in your oven. In ours, it's at the top right, in the middle. Always ensure there is sufficient space around it for the heat to hit it. If it's blocked in any way you'll find the oven just keeps pumping heat out. It'll be 300c at the bottom but the thermostat will still think it 100c.
Sometimes you will find the tops are now cooking perfectly but the bottoms are a little less done. At that point, you will be safe to either move the food down a shelf to help crisp the bottoms or if making scones you can safely flip them over just to finish off.
Good luck.
Best Answer
It sounds like you underbaked the cake to me, the structure hadn't crystallized and therefore couldn't support the weight of the case. Non-fan ovens often have warmer and cooler spots, the cake on the left was likely in a warmer spot than the one on the right, and cooked faster. Or the cake on the left was slightly smaller and cooked faster because of it, either way when you opened the oven door all the heat went out, lengthening the time it would take to finish baking the remaining cake and you just didn't give it enough time.
Next time test the doneness of the cake using the appropriate method for that cake. I use a spring test and/or an instant read thermometer for most cakes. I don't usually use a toothpick test as I've found it to be inaccurate.