The cause is the use of dh_strip_nondeterminism late in the build
process. This reorganises the jmod files, which in turn changes their
SHA256 checksums. This would not be a problem, except that the
checksums are saved in java.base.jmod *before* the use of
dh_strip_nondeterminism. Performing this stripping immediately after
each jmod file is created results in the checksums being consistent
throughout.