Just happened to me this morning. First time in 17 months of ownership from new. 2018 LCI M2. Right hand side only.
Will see if I can get the dealer to replace both tail lights with the dark line version from the M2 Comp
Suspect it could be water ingress from a seal in the boot. Had something similar on my 1M.
It’s been pretty dry and warm here in UK recently but we have had a couple of days of pretty heavy rain.