Apple has now explained why the new silicone ear tips for the second generation of AirPods Pro are not officially compatible with the original AirPods Pro.
In an updated support document explained Apple says that the original AirPods Pro ear tips have "significantly denser fabric" than the second generation ear tips. Apple has not provided any further details. But the mesh density could lead to acoustic differences. The support document states:
AirPods Pro 2 vs. AirPods Pro 1: Apple explains how to use the ear tips
The ear tips are specially designed for the AirPods Pro generation to provide the best possible sound experience. Therefore, use the ear tips that came with your AirPods Pro. The AirPods Pro (1st generation) ear tips have a significantly denser weave than the AirPods Pro (2nd generation) ear tips.
Despite the differences in the mesh, the second-generation ear tips fit the original AirPods Pro, so they are still physically compatible.

The second generation ear tips are available in a new XS option while the original ear tips are still only available in sizes small, medium and large. Both generations of ear tips are available in the Apple Online Store in sets of two for 10 euros in Germany. (Image: Apple)