Hi, all I also met this problem. I found that the problem involves imath and numpy.
After numpy 1:2.2.3+ds-3, the dh_numpy3 script added a behavior[1] to dig into debian/<binpkg-name> to look for shared libraries referring numpy capi, and only add the dependency if there is at least one. In imath, however, dh_numpy3 is invoked before dh_install [2]. As a result, python3-imath built from imath with numpy3 no older than 1:2.2.3+ds-3 will lack the dependency to python3-numpy. Opencv B-D python3-imath, and as a result, if it is built with a rebuilt version of imath, it will FTBFS with missing dependency to python3-numpy. However, the current version of imath in sid was built with 1:2.2.2+ds-2,and is not affected by the change of dh_numpy3, so trying to rebuild either package of imath opencv individually cannot detect this issue. Cheers, Miao Wang [1]: https://salsa.debian.org/python-team/packages/numpy/-/commit/507300db7472ffc437d60432c385394fcf21ffde [2]: https://sources.debian.org/src/imath/3.1.12-1/debian/rules#L32
