Setting fixeddeltatime depending on TimeScale makes physics behave weirdly

I ran into a Physics issue when setting the fixedeltatime timestep at runtime according to the current Time.TimeScale.
I frequently change the TimeSclae in my project at runtime in order to create slowmotion and bullet time effects. In order to also have smoothly working Physics and other features depending on the fixeddeltatime, I also set Time.fixedDeltaTime accordingly (Time.fixedDeltaTime = 0.02f * timescale).
While this indeed gives me smooth behaviour even during the slow motion I ran into related issue that I cannot seem to solve:
Applying a force to a rigidbody does not seem to work at all times anymore. This also occures if the TimeScale is 1 and thus fixeddeltatime = 0.02f.

I tried to debug the problem and it seems like it is related to setting fixeddeltatime at runtime, regardless of the new fixeddeltatime value. This problem even occures when time fixeddeltatime is only set once at the start to the default value of 0.02f.
In short: setting fixeddeltatime manually (even setting it once at the start of the game to the same default value of 0.02f as it already is makes physics work in a weird way, which leads to forces sometimes not being applied properly)

Does anybody have an idea how to resolve this issue?
If need I can provide video material and code examples

Don’t change fixedDeltaTime. Doing so will change the physics behavior in unpredictable ways. Change Time.timeScale alone to get physically coherent behavior regardless how often timeScale is changed, or its value.

You can get visually smooth motion at any time scale by enabling Interpolation in the rigidbody.

I’m used to create slow motion (or fast motion) effects, and that’s the correct way to get physically correct results independently of the actual time scale.