Hi there, I've traced inter-package links (depends, conflicts et al) and I think I found the reason why the server gets removed.
stretch still has a mysql-common package that has a "Replaces:" for mysql-server-5.5. during a dist-upgrade mysql-common will get upgraded to the stretch version, which in turn will remove the server package. to test this hypothesis, I ran "apt-mark hold mysql-common" right before doing a dist-upgrade from jessie to stretch and both mysql-server and mysql-server-5.5 then don't get removed by apt/dpkg.
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