Positional matchmaking and promos

-Ok, so I am a B4 Mid and Jg main. I finished last season at B1 and my highest rank was S5. So I started this season and got placed B4 in both my roles. Ok, fine. I started to climb. In the last week or so I reached 100 LP on both Mid and Jg. However, in both roles, I actually got to 100 LP through splashing. At the time I thought it was awesome, "Cool I don't have to play another match for the 3-5 LP I need to get to 100". I thought I was going to get to start my promos right away. That wasn't the case. -The first role I got was Jg. I won the game and expected to get my first promo win. But instead, I received 0 LP and got into my promos. It felt really bad to win a game to and to get no advancement out of it. -In contrast to that today I finally played my Mid game at 100 LP and lost. So now I'm down to 84 LP without getting to play promo games even though I got to 100 LP. I don't know about anyone else but to me, it feels really bad to either 1: Get to 100 LP through splashing LP, win a game and receive no reward/advancement. 2: Get to 100 LP through splashing LP, lose a game and not get to play promos at all. Is this system meant to work this way? If so I would like to ask Riot to please consider changing this as I'm sure I'm not the only one who has felt this way.

P.S. Riot I do love the rest of positional matchmaking and want to thank you wholeheartedly for it. Keep up the good work!