Custom Gas Thrusters Can Run on Hydrogen and Vice Versa
The issue was described well here: https://support.keenswh.com/spaceengineers/pc/topic/normal-hydrogen-thrusters-using-custom-gas
but unfortunately this "outdated" ticket did not get any attention from the support team. It seems creating a new ticket works a lot better, so I'll try to summarise the problem again here.
There are many good mods out there that add new gases and new thruster blocks that use those gases as fuel. They work analogous to hydrogen thrusters in the vanilla game. However, there is currently a bug in the game that, based on certain conditions, causes hydrogen thrusters and these custom gas-powered thrusters to start using each other's fuel, and even become unable to use their own fuel.
In order to reproduce this, you will need to create some custom blocks and a custom gas, as if you're building your own mod. Or you can use an existing mod as I mention at the end of this post.
1) Define your own gas, a gas tank block that can store this gas and a thruster block that uses this gas as fuel.
2) Load your "mod" into a new creative game on a planet (so you can build grids on the ground).
3) Build a new grid on the ground. Build your custom thruster and custom tank on it. Make sure they are connected by conveyor.
4) On a separate new grid, build a hydrogen thruster and hydrogen tank. Make sure they are connected.
5) Put the thrusters on a tiny thrust override to observe that both of these systems work fine on their own grids, properly drawing fuel out of their own tanks as expected.
6) Now, on the first grid where your custom gas thruster is, add a hydrogen thruster and hydrogen tank. Make sure they are connected by conveyor to the already existing custom gas tank and thruster so they're all on the same conveyor line. Make sure both thrusters are on thrust override.
7) Observe the fuel draw from the custom gas tank become faster and the hydrogen tank showing no fuel being consumed, as the hydrogen thruster is now also (somehow!?) running on your custom gas.
8) Now re-do steps 6 and 7, but the exact opposite way: add a custom gas thruster and tank to your hydrogen thruster grid. Now observe the custom thruster running on hydrogen!
9) The last weird thing you can observe is to turn off the tank that is supplying the fuel. You will notice that all thrusters will cut off and even though there are tanks with the normally correct fuel on and available to draw from, the thrusters will refuse to do so, in effect rejecting to use their own fuel type.
As you can see, when each system is isolated on a separate grid, all works fine. But major problems and exploits arise when you try to use both systems on the same grid. It doesn't work as intended and you can even consider it a form of cheating if you have this happening on a modded server with custom thrusters.
If you don't want to spend time defining your own custom gas/thruster/tank, and you're ok with using an existing mod in your testing, I mentioned here a mod and blocks you can readily use instead to reproduce the problem.
I hope you finally give some attention to this bug.