I've been wondering myself. I've done about 8 runs on 90/12 and perfect drops every time. Definitely better rate than the Ryujin invasion.
And concerning that 5 stam map. If the drop rate was at least 50% it would be identical per stam to the last map (plus or minus RNG).
You also get the Gadoras making it even more worth it. I wonder if anyone is keeping track of drop rates cause 50% doesn't seem too unreasonable.