As concluded in the PR discussion this is not a bug and introducing that behavior to Symfony cache:clear command is not a good choice.
Since the issue is LTS-only, what we can do is to improve the Doc stating that in case of Redis its cache has to be cleared using dedicated command line application: redis-cli flushall.
EZP-28903 for that.
Of course for 2.x we could also prepare a dedicated command to clear all Content cache (HTTP cache + Persistence cache). It needs to be taken care as a new story for a Sprint.