The attached datapack contains a suite of advancements that use functions and /schedule to remove themselves after a certain delay–immediately, after a 1 tick schedule, and after a 2 ticks schedule, one set having display information, and the other set not.

To reproduce, load the datapack in singleplayer and grant yourself each of the advancements in the test namespace in turn. When removing the 1-tick advancement with no display, the game output will receive a warning: "Told to remove advancement test:no_display_1t but I don't know what that is".

Given the output of the other advancements (the display and immediate no-display advancements give no messages, while the 2-tick no-display advancement says the game "Forgot about advancement test:no_display_2t") I'm guessing that this has to do with the advancement tree–an advancement that removes itself in its own function reward never tries to add itself to the tree, but 1t scheduling bypasses that and causes the game to try to remove the advancement from the tree before it's actually been added to it.