I believe this is a config workflow bug.
If you've ever enabled mesh, it can't fucntion with spectrum-monitoring enabled, so if mesh mode was ever activated, then this option is disabled regardless if mesh mode isn't currently active or what you do in the GUI or CLI -- in fact, the CLI shows it enabled.
The solution to get the feature back was to reset the config to an earlier state. But that also disabled the ability to enter mesh mode.
It seems that there are several pre-requisites for mesh mode to function which are inconsistantly documented. e.g. Extended SSID must be disabled, spectrum monitoring disabled (5.0GHz), and a few other requirements which must be manually set before it all works. All of the docs for mesh mode cover it from a fresh factory config but if you're trying to enable mesh later after you've been tweaking settings, it's a hair-pulling effort to get everything setup to get mesh enabled.
In fact, in one document it mentions it won't work with 802.11ac enabled when in current releases it works.
IMHO, the mesh config mode should have its own config screen and help texts updated to link the mode dependancies a bit better instead of it being a scavanger hunt for setting correct options.