I first encountered an issue like this one when producing the very first ROS 2 beta 2 debs shortly after joining Open Robotics (and I'm sad to say that in all this time I haven't taken the time to investigate the issue to my satisfaction. We ended up adding patches in the bloom-generated debian/rules.em templates which explicitly add the directory containing the "missing" library to the dh_shlibdeps search string.
I think a patch similar to the one above to the debian/rules.em template file in your bloom repository will resolve the issue but I don't have a deeper understanding of the problem to know if there's a "better" solution out there. If I ever dive into this mystery I will come back and update the answer.