Mem used consistently higher than high water mark but no eviction

we have a couchbase cluster v4,
we have total 2 buckets, in one of them we receive constantly: hard out of memory error… all memory allocated to this bucket is used for metadata.
at this bucket there is very little activity (6 ops per sec on average) and the memory used is above the high watermark, however no eviction occurs.
ATM: the used memory is over 92% of the quota and high watermark is set to be 85% by default

is it possible that almost none of the resident items can be marked as NRU hence can not be evicted?