Could be green wood I have the same issue with my 2015 SG unplayable. I pulled out my 2001 Custom Shop Les Paul it's been in the case since 2002 still in tune and stayed in tune.
After Gibson got raided who knows what wood they are using.
View attachment 13049
I have to say that my 2016 Gibson SG T Series is totally stable from a tuning stability standpoint. It impressed me with that feature.