This is entirely server-side UA sniffing going wrong. You get an empty HTML doc, only a doctype, with a Firefox Android UA. You can reproduce this with curl and it seems that this affects all UA strings with versions >= 65. <=64 work.
It's interesting if you remove "Android" from the UA, then it sends more, but remove "Mobile" it still sends only the "doctype", and removing "Firefox" fixes it entirely.