i am also 6-5 and had this exact same problem. get an anatomy textbook and look at your hips. the reason that you lose your arch is lack of flexibility and or/strength in the hamstrings. you can most likely keep your arch in say-rack pulls, but off the floor you can not maintain it-right?
most taller guys have strong low backs and weak (relatively) hamstrings. your hams are tight so your hips rotate downward, and you waste all of your low back strength fighting your hamstrings to keep an arch. with significant weight your low back simply can not pull both the weight and your hams!
get Pavel’s stretching book, it explains all of this. it took me about two months of consistent daily stretching (yoga) to get enough active flexibility to dead properly. it was well worth it though. i was struggling with 405x1 about 6 mo ago-last week i pulled 405x10, after my heavy strongman stuff!