Unexpectedly high non-mapped virtual memory usage on secondary

Upon upgrading a secondary from 2.6.3 to 2.8.0rc2 mmapv1, MMS began reporting a consistent large increase in non-mapped virtual memory usage: from ~1.2GB to ~19.4GB (screenshot attached).

MMS defines non-mapped virtual memory as:
(serverStatus.mem.virtual - 2 * serverStatus.mem.mapped)

The 2x is applied because journalling is enabled. (And was enabled both before and after the upgrade.)

Open connections reported by this host also remained the same before and after the upgrade.

For easy reference, below is the serverStatus.mem section of this host's last ping:

        "mem": {

            "virtual": 576484,

            "bits": 64,

            "supported": true,

            "mappedWithJournal": 556650,

            "mapped": 278325,

            "resident": 34019

        },

# MMS' calculation:

# (576484 - (2 * 278325))/1024 => ~19.4GB