The XP you get from sending mobs applies to that match only. We'll refer to this as "mob XP."
The XP you get from saying "gg" applies after the game (to your level progression). We'll refer to this as "player XP."
When you send mobs, you get mob XP that can be used to buy upgrades. When you say "gg" or "wp," you get 1 player XP from the match but only if you say it when the game ends. You get the match's full player XP only if you win (so 125 player XP for winning, add 1 if you say gg at the end of the match). This is not a bug.