Thanks for the tip @mortac8. WRT1200AC indeed uses the 88W8864 radio chip.
I've just setup my router with the configuration you propose. I'll report back here the results on the long run.
If it works, would it confirm that this is a mwlwifi firmware issue?
There is a known issue with WRT32xx which seems very closely related; downgrading the mwlwifi firmware may be an acceptable fix.
Does anyone know if there will be further development on this topic?