LP bug after winning

i just recently got promoted to gold 3 and as such started with 0 LP if i am to assume correctly. after playing my first mach in gold i received +18 LP and that put me at a total of 15LP. as you can see there is 3 missing LP i should be at 18, im not mad i just figured i should post it in here and im not sure if this has happened before, so far it has been just the one time that i can remember will continue to watch.