I found the problem. The public headers of the UnityFramework.framework that is built just before the main app aren't copied in the framework directory (I thought it made no point to embed source code headers when doing a binary build) but it appears the main app expects those headers to be here.

The fix is on the way.

:: @Pierre-Marie Baty added on 29 Feb ’24 · 10:07

*edit* Version 3.62 is out, and it builds your project successfully :-)

The real problem was that the public headers of a just built framework were not embedded with the framework in case the framework had no Swift module definition file. Since UnityFramework is an Objective-C framework it has no such definition file. But its Headers directory needed to be populated nonetheless after the build for the main game binary depending on it to build.