UPDATE: WARNING!
After further testing, I noticed that even after having done the above, if you allow guests to book more than 27 nights, the darn arbitrary and unauthorised monthly discount activates itself again and you have to manually remove the custom monthly price again!
Whether this is a glitch in Airbnb's programming or an intentional 'bug' of some sort, I can't say. Having flagged it to Airbnb tech and getting no response, who knows.
In short, to be assured of unauthorised massive discount being quoted to your guest for monthly or longer stays beyond 27 nights, you will have to adjust Trip Length settings:
1. Set maximum stay to 27 nights (if 28, it auto triggers the monthly discount)
2. Not receive request for longer stays beyond 27 nights
(See screengrab below)
But what to do if a guest wants to stay more than 27 nights?
If a guest wants to stay, say, 29 nights, you may have to get them to book the extra two nights as an additional booking, or maybe it will be possible to change the offering to the guest in terms of both agreed rates and time frame. (Like what you would do it if a guest asks to change reservation dates or asks for a special quote that's different from original price.) However, I haven't tried the latter, so not sure if this circumvents auto monthly discount from activating.
Good luck everyone!