Hi there,
I have been struggling with light leaks similar to OP’s for a month. I have read through the whole forum, including the let’s make lightmass epic topic, tried to increase various variables in the config file, disabled MikkT tanget space, tried to delete the Lightmass cache that @RyanB suggested, but none of these helped removing the artifacts completely. I’m using 4.18 but even with the new multibounce skylight this kitchen was super dark so I had to hack it quite a bit. Effectively the main lighsource is a spotlight with a bounce card (half sphere in my case as I found it to have better coverage). I also had to add a direct spotlight because the walls were noisy at the door (still are but less noticable). Found this quite strange as it’s not a distant small hole so it should get enough photons reaching it. I have attached screenshots of 2 variants, both using the same Lightmass settings. The darker one uses the default lightmass ini file (bake time 7,5 hours for the whole house). The brighter one has these variables modified (bake time 15 hours for the whole house):
[DevOptions.ImportanceTracing]
NumHemisphereSamples=32
[DevOptions.PhotonMapping]
NumIrradianceCalculationPhotons=2000
DirectPhotonDensity=1600
DirectIrradiancePhotonDensity=1600
IndirectPhotonDensity=1200
IndirectIrradiancePhotonDensity=2000
IndirectPhotonSearchDistance=2000
I have completely run out of ideas so I would greatly appreciate if anybody could give me a helping hand with the matter. 
Regards,
Fisher