First, let me start by stating I'm in no way trying to tell you guys you're wrong, this is intended to be a helpful post. There's enough people talking about this that I believe something may be going on.
Issue 1) Which a lot of people are experiencing, is the lack of downshift when using paddle shifters at or near maximum revs. This is an over zealous DSP (Down Shift Protection) mechanism cutting in. I get this regularly and some cars are worse than others (sometimes).
I think this is part of what's confusing us PC guys. This is exactly how DSP is supposed to work, to prevent you from downshifting into a lower gear when the engine revs are too high. This is how DSP works in real life, any car with a proper paddle shifter won't let you downshift if the engine is at a speed that would cause damage by going down to a lower gear. So IMO, based on this limited description, it does not sound like an issue or that it's being overzealous, it is working as intended. When they first introduced DSP to AC I hated it and was missing shifts all the time, after a week or two I got used to it and now I don't even think about it. I still encounter it a few times a week, but it's pretty rare because I've adjusted my driving style. Last night I was testing the Maserati GT4 and actually had it happen several times in just a few laps, but I also drove the Cayman GT4 Clubsport at the same track and did not encounter it at all.
Issue 2) This is the one that some people have experienced (me included) where the car will not downshift until you are going very slow and are at approximately half revs or lower. Makes the car almost undriveable and this is what I have reported the possible fix for here. If you have had this you will definitely know you had it. It is that bad.
This, however, does sound like an issue. As long as you're at a sensible engine speed, say three-quarters rev or lower, it should always let you downshift. Forcing you to wait until you're at half revs or lower does seem overzealous and that suggests to me there's something screwy somewhere. There are a few cars on the PC that are more restrictive than others (Alfa 4C comes to mind, last I drove it seemed like you had to be near half revs to downshift) but the majority of the cars will let you downshift with much higher revs than that.
Another issue that needs to be addressed here is that Consoles and PCs are on totally different software and what happens in one doesn't necessarily transpose to the other. Even though consoles are supposed to be a few updates behind PCs it still isn't the same. So having PC guys (as helpful as they think they are) telling us we don't have a problem or in some way imagining it is not very helpful or productive. Of course PS4 and XBone are slightly different as well and the XBone isn't quite as powerful as the PS4 so also introducing issues.
I hope this clears the air a little ;-)
This is why I've not chimed in much. I've not played the console version so I only know what I'm reading and I haven't seen enough specifics or videos of this problem to be of any help or form a solid opinion of what's happening. DSP is restrictive and will not let you downshift if you're at very high revs (some cars are more restrictive than others), when this feature was first added it caused a lot of confusion and complaints on the PC side as well but now it's never even discussed because we've all just adapted to it. So my first thought was that you guys were going through the same cycle, but it does seem like there's possibly an error happening in some instances.
If you are interested I would be more than happy to do some tests on the PC, make note of when certain cars downshift/won't downshift, and then we can compare the same car/track on console to see how the results are matching up. That will give us hard evidence to look and compare how the different version are behaving. Just let me know if I can do anything to help, happy to do what I can.
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