I do put it to sleep; I guess I missed adding to "I leave it on for days at a time without shutting it off." I should also add that I have it set to sleep, and have no issues at all waking it from sleep. I did turn off "Wake for network access" as I do have my system networked with other devices, servers etc. in the house and always turn that off thinking I don't want it to wake in the middle of the night for any 'wake' packets that may be sent to the system.
I wonder if some exchanged their card for a new W5700X? It's weird that some have the issue and some don't. Sort of looks like some software incompatibility somewhere which is probably really hard to chase down as everyone has some different configuration of software, peripherals, accounts, maybe memory setups etc.'
EDIT: FYI it's not just Mac guys with the W5700X having issues either
https://www.pcgamesn.com/amd/radeon-big-navi-driver-issues-fix
IMO I do wish I could justify the extra money on the Vega II - mine seems stable enough with my configuration, otherwise the first thing I'd do is force a return of the card and/or computer on the whole and obtain one of them Vega cards as they seem much more reliable in conjunction with the drivers.