As discovered when investigating EZP-28816, the fact that Symfony cache:clear doesn't clear Redis Persistence cache (or any Persistence cache as a matter of fact) is the expected behavior.

Since the issue occurs in LTS, the one thing we could do is to place a note / comment on the Doc (somewhere where Redis setup is mentioned) that when using Redis, its persistence cache needs to be cleared using console command shipped with Redis:

redis-cli flushall

The exact location of that command and thus execution call depends on local server setup.