during a test with piuparts I noticed your package failed to properly
upgrade from stretch. In some cases the old hfst package from stretch is
kept installed in order to keep the obsolete libhfst45-dev installed.
apt does not upgrade from virtual packages to real packages.
A better solution would have been to re-introduce a libhfst45-dev
transitional package, but it's probably too late to do this in a buster
point release. Adding the Breaks should be easy in a buster point