I have friend in Pta with cracked frame at the back on sub assembly.
He carries a Givi top box on a Yam genuine part frame thingy....he confessed that he loads it bit more than what is written on box...so we suspect that might be cause.
Crack is somewhere in regoin of exhaust bracket.
Are you loading anything on back when doing these trips?,,,maybe you must support those fuel tanks towards the centre of the bike at bottom. did that with my panniers...ran a bar down towards the footrest to give support low down to middle of bike instead of carrying all weight on frame at back.