Update (for anyone using the unofficial OctoLauncher Linux rebuild, not the official client): this is fixed as of the latest release (v1.3.10).
Turned out to be a genuine race condition in the WoW.exe client itself — one of its own startup routines isn't properly thread-safe, and it practically never loses that race on typical desktop CPUs, but higher-core-count hardware changes the timing enough to hit it reliably. The launcher now automatically pins the game to a single CPU core to work around it, which is safe since the 1.12 client never needed more than that anyway. No settings to change, it's on by default.
Grab the latest build of the unofficial launcher here:
https://github.com/OxLlamaLinOct/octola ... ses/latest