Reading between the lines a bit, it sounds like the problem is that the teams have put so much time and effort (and money) into channelling the exhaust gasses over the diffuser to generate downforce that they’ve overlooked the way the tyres naturally deform when load is applied to them. This is something that happens to any tyre – as the air pressure increases, the tyres expand. Likewise, as more downforce is applied, the tyres react to it by expanding.
So I’m guessing that the problem is that the teams have arranged their exhaust flows to get as much downforce as possible, but as the tyre naturally expands and contracts under load, it’s interrupting the way those exhaust gases reach the diffuser, either preventing them from passing over the diffuser, or forcing them to hit the diffuser at a less-than-optimal angle, which is interfering with the level of downforce.
That’s not a tyre problem. That’s a team problem. They’ve become so concerned with the relentless pursuit of downforce that they’ve committed a massive oversight and forgotten abotu the tyres.