Does symfony 2.8 load bundles and services in correct order during warmup?
Just try moving your initialization code out of the constructor (I know you already know that). Symfony does more during the cache warmup process than it did in 2.7. This service is instantiated likely due the Twig templates being warmed up. The probably issue is that Propel itself probably needs some cache to exist in order for it to be booted... or perhaps there's some code that helps boot Propel correctly during the request-response cycle.
In short: this looks like a quirk of Propel to me, but someone who knows more about Propel might know a way to forcefully "boot" it. Propel is special because of the static context it uses.
I hope that clarifies a little!