Ok, that gave me a more clear picture of your Rack, and situation, i just got caught up by
So my thought was the that this nearby device, maybe not was build to handle a huge bunch of children, and therefore was part of your problem.
Beside the “Rack” looked like a busy server-rack with a disk-cabinet etc. 
, also i was not aware you had so many “repeaters” in your zigbee mesh, i still have a very small amount of device, compared, so with the various Topics i’ve seen here , beside the Various hardware(Coordinators), i came to think about that (“Mesh” in all honor), but my new wifi-router supports wifi-mesh, and 2 plugs can act as zigbee repeaters, but i have never been a fan of the popular Mesh(term), due to the fact i still stick to old proven tech. , and not have looked deeper into the “new” Mesh tech, other than noticed the various problems people seems to experience, like less control over the route(in particular), confused devices, etc.
Hope you “narrow” down your issue soon, … i think i will invest in a “signal analyzer” if (when) my system grows to large , but i definitely got many hint’s, cons and pros since starting here in this Forum
PS: in regards to LQI, i have atleast 7 devices working fine beneath 70-80