Facebook dropping MSS on congestion

Good day,

I am writing here, as in technical support ticket I will most likely end up to the outsourcing guys, who will try to write some formal reply and close the ticket quickly to keep KPI high:)
I have a faint hope that someone will read and listen. It may also be useful to colleagues.
I noticed at last few month, if some congestion occurs on the network (specific subnet), facebook reduces the maximum segment size (MSS), even down to 256 bytes. Purely academically, on paper - this will reduce latency.
In reality - it will cause avalanche effect.
If ISP have CGNAT, or some other appliances - with great probability they will encounter the fact that pps will increase 4-5 times, and might hit pps limit on hardware. Additionally, overhead on IP headers will increase significantly, especially on ipv6, and this will further aggravate the congestion.
Facebook don't do that, please. And thank you, if you listen to suggestions.