I triple checked this in mwdebug2001.
In BagOStuff::set(), the documentation clearly says @param int $exptime Either an interval in seconds or a unix timestamp for expiry. As result, most of Wikibase normalizes TTL to unix timestamp for the expiry (See SimpleCacheWithBagOStuff::normalizeTtl). Its implementations however doesn't seem to care about unix timestamp.
For example. MediumSpecificBagOStuff::set() just passes that to doSet() and for example APCUBagOStuff::doSet() directly feeds that to apcu_store and documentation for apcu_store clearly state it's TTL in seconds, meaning it sets it for that amount of unix time in future meaning the cache value will expire roughly around 2073.
Memcached and most other implementation seems to parse unix timestamps properly but wincache doesn't seem so.
Maybe we can check how many are set like this in APCu if possible. We need to fix this and also clean up APCus across the cluster. (That's why I tagged SRE)