Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: AMD Mobility Radeon HD 5000 Series ATI Technologies Inc. 4.5.13399 Core Profile Context 15.200.1062.1004
Broken: version: 2.90 (sub 4), branch: master, commit date: 2020-05-28 18:34, hash: rB2deb6c517787
Worked: (newest version of Blender that worked as expected)
Short description of error
This is not exactly an error, is such an unexpected harmless behaviour.
When you setup a scene for mantaflow (liquid), and you change from liquid to gas the domain type, after you bring back the gas to liquid, it won't auto re-enable the liquid checkbox (if you don't check it, mantaflow won't correctly bake the information).
It's really a super small thing that is of course not priority. But for GUI consistency it would be good to fix this since when you create the domain with liquid it has automatically checked the liquid checkbox.
Exact steps for others to reproduce the error
Change domain type from liquid to gas and from gas to liquid. You will see that the checkbox won't be auto-checked.