Update: I worked with an InfluxData dev on this issue and it's indeed upstream. gopsutil[1], a golang dependency for Telegraf, needed to be fixed, and a gopsutil merge request has been approved upstream.
I compiled Telegraf-1.6.0-rc4 with the patched gopsutil dependency and it works like a charm on all 3 instances. However, the gopsutil fix will not be in the Telegraf 1.6.0 final release, rather, it's been slated for inclusion for the Telegraf 1.7.0 milestone.
Also, as of Telegraf 1.6.0(rc-n), there are a host of new plugin and external golang dependencies. So I'm not sure how the pfSense Telegraf package maintainer would like to proceed.
I have the updated Telegraf Makefile and distinfo, declaring all the additional and new deps, as well as the proper GH commits etc. to make this all work for the RC and forthcoming 1.6.0 Telegraf release if the maintainer would like them.
[1] https://github.com/shirou/gopsutil