System Information
Operating system: Linux-5.4.0-47-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.51.06

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-14 18:55, hash: rBf7829787da5c
Worked: 2.7x, 2.8x and 2.9x all

Short description of error
The Image Editor's slots aren't saved any more like before.

Exact steps for others to reproduce the error

• Open Blender (Factory Defaults) and press F12 to render the default cube.
• In the Image Editor that displays the current render switch to another slot, e.g. by pressing 2 on the keyboard in case you rendered in slot 1
• Move the cube or do whatever you want to change the render result in any way
• Press F12 again
• The new render looks different. Now switch back to the previous slot: It no longer contains the previous rendering.