The post-match timer for adding GGs doesn't always show for me. The game just skips right to showing the total percentage everyone else added without giving me a chance to add GGs. Happens about 50 percent of the time, but only after PvP matches, never in vs Bots.

It doesn't matter if I do or don't add GGs during the match, and I doubt it's some sort of latency effect due to the length of the timer.