Found a discussion here which might explain some things.
@GieltjE: On tweakers.net, you stated (translation) as soon as de Redis cache provides an expired record, a new query is performed. This appears to be correct, but the request is (see below) performed after the response is provided to the client, this to update the expired entry (see below - prefetch=yes).
I, and probably others, added prefetch=yes to my unbound config, as the result of a discussion (@DL6ER - Everyone of the team agreed to remove the lines) , where it was decided to remove cache-min-ttl and cache-max-ttl from the recommended unbound config, so the behavior @GieltjE has seen is probably because of the prefetch=yes setting.
The expired record will be served to the client only, if the setting serve-expired: yes is configured, as pointed out by @anon55913113.
So I changed my unbound config, ensuring the following settings are already there or added, and there are no cache-xxx-ttl settings in the config.:
prefetch: yes
serve-expired: yes
serve-expired-ttl: 0
serve-expired-ttl-reset: yes
Some questions remain, regarding the use of Redis, as pointed out by @DL6ER (I still don’t see the need to add more moving gears)
Restarted unbound, emptied the Redis cache (redis-cli FLUSHALL), so it's to soon to tell what the result will be, however, it remains fascinating and worth discussing...