Was the red player out of the game already at this point? i.e. were his pieces except for his king greyed out already?
20 or 25?

No, they both were alive. I checkmated the yellow and after a couple of moves the red stalemated himself.

OK, so both were alive. Was one (or both) of them left with only the king, without any other pieces?

No other pieces, they had the kings only. Please explain this: "when only king is left every move is forcing." Do not understand your idea.

Thanks, Bab, I think with this the programmers may have the info needed about this.
I remember that recently there was some talking about avoiding the possibility of collecting endless points giving double checks including an untaken king with greyed out pieces. If they recently did a fix about this, this may or may not have affected (intentionally or unintentionally) what is happening about double checks in scenarios such as yours here.

some relevant posts from here:
I think the current way it works with double check which includes a player that resigned/timed out is weird and should be removed. I saw a game where 2 players were remaining, but there was also a king left from a player that resigned. Normally trying to capture that king as quick as possible is a good idea, but it was a queen ending where the player with initiative could create a lot of checks against the enemy king which included double checks against both kings. After being behind almost 20 points the score lead quickly shifted and when he finally stopped giving checks and took the king the game was pretty much over.
Okay @Martin0, that was left in intentionally, but we will remove the double checks on kings from players already out of the game.

One post I wish is relevant, but I haven't heard anything about what others think about this idea. Most likely (almost 100% sure) not relevant though.
One more thought. Maybe triple checks should be worth 20 points and maybe there should be a limit how many times you can get points for a double and triple check. Like each player can get points for a maximum of 4 double checks and 1 triple check. This is to prevent infinite points scenarios.
In the position below I moved my rook to the D rank, as a result the yellow king was checkmated and the red one checked. So, it was a double check + checkmate at the same time. I believe the server added +20 to the score at that moment. Should it be +25 or +20?
P.S. For the testing purpose it would be good to see the current score (not the final one) in the game replay mode. Of course it is not very important, but could help to find more bugs.