Welcome to the Icinga community and thanks for posting.
Based on the icingadb-redis-journalctl.log log file, I would assume that your system simply runs out of memory and Redis is the first fallout.
Sep 12 23:00:08 monitoring-master-01.company.tld icingadb-redis-server[1268522]: 1268522:M 12 Sep 2025 23:00:08.815 # Out Of Memory allocating 86032 bytes!
Never exceeding 90% of memory usage is one thing, but how does it normally look? Can you please provide a report/graph of the memory consumption through an hour before the next Redis OOM crash?
And is it really only Redis to go OOM? Based on your statement of 90% memory consumption, I would expect other processes to also get killed.
On a similar question: Do you have the OS OOM killer configured?
As an alternative, you can try to tweak the save option in the redis.conf, as mentioned in our docs, https://icinga.com/docs/icinga-db/latest/doc/07-Operations/#huge-memory-footprint-and-io-usage-in-large-setups.