Solved

Tires bug

How hard tires could be faster than soft ones on 10ºC, same weight, same setup... etc.
Does it a bug?

They only become faster than Softs once the Soft tyres fall below 50% or the Soft tyres are overheating. When both tyres are running at optimal temperature, then Softs will always be faster until they reach below 50%.

Yep that's the rule of thumb for tyres atm, even though I've also been saying that Hards should be graining in such low temps! Something that can be tweaked first regarding the tyre balance is the tyre warming especially for Hards in cold temps.

To go into more detail about the assessment, the amount you can push on tyres also greatly affects its pace. If S are running in optimal temps but you can barely go push 2-3 on them without overheating them, they will be slower than H which can go on push 5.

It depends on the track as well. In Brazil in 20C I notice Hards are definitely faster than Softs (like almost half a second faster per lap through the second half of a 13-14 lap stint each) even if both are running on optimal temps, simply because you can push harder on Hards while Softs are easier to overheat. It's 20C so it's not as bad, but shows the strength of Hards on tracks such as Brazil.

Whereas in Germany I've been in a race where H and S have equal pace in 10C, yet towards the end, probably when S is at around 70% tyre life onwards, don't exactly remember, the H becomes around 0.1-0.3s per lap faster. This happened in the 1 car league IGP Champions last season, where I was one of the S runners, and one of the great Romanian managers there was on H. The H runner was faster than the S runners by around the margin I stated above. Those I race against tend to say I have great pace too, so I believe I was optimizing the pace of those S as much as I can, but others may have different results when they try it out. Always something new to learn.

The European tracks are still where you can find the most variety of competitive strats, though. It's just overall throughout the season, H is seen as the default race tyre on most tracks (S can be the race tyre on some European tracks, and Aus), with SS as the sprint or undercut tyres (and also competitive race tyres in Aus, Hun, Eur, and Bel) at the right stint length. Most of the variety in strategy then comes from which tyres you start on, and how many stops you have as well as your stint lengths.

One track where I have seen Hards be outright worse than S in cold temps though, apart from Monaco (in which only SS works there), is Hungary. But someone may come up with a strategy on Hards which could work in cold Hungary, who knows (Hards work in warm or hot Hungary).