(In the last 10 minutes from writing this) I sent a 1-check Fog-of-War challenge, my opponent saw it as a one-check (also FOW) challenge (at least they claim to have seen that in the chat, I cannot independently verify it), but when I delivered the first check of the game, it just reduced 5 to 4.
https://www.chess.com/variants/custom/game/15418241/16/3
I refreshed the tab, and it still shows 1-check in the match-type name and on the banner with the list of rules that are combined (although once one hovers over it, it describes the rule as how it is in 5-check)