Ephemeral buckets: item count goes to -1

Running on build 2412. I have test runner test:

./testrunner -i 1node.ini  -t eviction.evictionkv.EphemeralBucketsOOM.test_ephemeral_bucket_NRU_eviction,replicas=0,bucket_size=100,items=1000,vbuckets=1024,bucket_type=ephemeral

This test creates a 100M ephemeral bucket and add items in batches of 1000 until 90M of memory is used. I see that when the bucket is empty 47M is used - this not believed to be an issue.

The bucket grows to 89M and then items are deleted to 74M and then it grows in steps to 104M bytes used at which point the cur_items goes to 64 bit -1, decimal 

18446744073709493681.

Logs are attached, please let me know if I can provide more information.