This happened to me only after I replaced the rocket with another one. I have Gas Rocket Port controlled by automation and piping oxygen into it, but it seem to just disappear for now.
Attached save file but after game restart it works.
Looks like the following:
1. Land rocket on platform with Gas Port
2. Disassemble it
3. Create new, different rocket
4. Start pumping oxygen
5. Get no oxygen in tank