Dear maintainers, I've faced with the same issue when tried to upgrade to the latest redis (and using old sentinel.conf). I think, this is because of this change: https://salsa.debian.org/lamby/pkg-redis/commit/bd244d314fe292307336f5815c2321f3cfe690c8#f8f779ca05364775b288799d69c4e2ad6278009a_76_83
I've fixed the issue locally in /etc/redis/sentinel.conf by changing old default "pidfile /var/run/redis/redis-sentinel.pid" to new one "pidfile /var/run/sentinel/redis-sentinel.pid". Maybe it will be useful to someone else. :-)
