as you can see from the screenshot I went +18 -> 36 lp https://na.op.gg/summoner/userName=ThronesLegend I've been winning a lot of [ranked] games, don't have proof of my lp gain before promotion but with this many amount of wins / high mmr it's indicative that my lp gain should be at least 20
edit: won another game, still +18 (now at 54 lp)
D5 generally has a higher mmr gap to get to D4, thus resulting in sudden lp drops.
Lp gains depend on your mmr relative to the average of your division, so peoples mmr gains usually drop (or 'only' remain the same despite the bunch of wins involved in the promo series) after promotions.
edit: Also keep in mind op.gg's estimate of your mmr (and the average mmr of a given division) are just estimates, so if your LP gains don't line up with op.gg's numbers are suggesting you ouaght to be getting, it probably means that op.gg's estimate is off.
Yeah, well rito figures since you're play freeloplank that they need to start capping yo shit.
