In this answer it is stated that:
If the fuel fully decomposes then return to Earth is still possible but only via ballistic descent
I find it a little bit surprising, but maybe there is enough Δv in the cold gas thrusters?
Most thrusters on the Soyuz-MS spacecraft use Nitrogen Tetroxide1 (N2O4, oxidizer) and Unsymmetrical Dimethylhydrazine2 (UDMH, fuel). These are stored (and burned) on the service module and both are fairly stable (from a storage-lifetime standpoint). For safety reasons it was decided that the descent module would use Hydrogen Peroxide3 (H2O2) monopropellent4 as its fuel. Most operations including orientation while on-orbit as well as the deorbit burn are carried out using the service module's engines (N2O4/UDMH); the only thing that the peroxide engines in the descent module are used for is controlling the module's attitude and thus its flight path in the atmosphere5.
1 Nasty stuff
2 Even worse
3 Not that nice either. However, the product of its combustion is water and oxygen which is also what it degrades to over time. This means that any leaks into the crewed descent module wouldn't be too bad.
4 The peroxide spontaneous combusts on contact with a catalyst (platinum for Soyuz-MS IIRC). This typically makes the thrusters more reliable since only the fuel valve needs to work for the thruster to function.
5 The descent module's trajectory can be changed by adjusting the heat shield's angle of attack against the atmosphere. Said adjustment is done with the peroxide thrusters. If the thrusters are not available then the angle of attack can't be controlled and the reëntry will be a bit rough (up to 8.2G rough).