I have never had rl26 act like that, but I have never loaded it in a 240 wby either. But in several 7 mags, a .260ai, a few 6.5 cm's, a few 6.5x284's, A .264 wm, a couple .270 Shermans, a .270, and two .338 Norma mags it acted rather predictable with normal pressure signs appearing as you got up there. However the improved chamberings move up faster, I think it's because they hide pressure better, soon as you see that first ejector mark you better slow your roll. Wonder if there was something else? Maybe accidentally loaded 57.5 instead of 55.7......I have made a mistake like that before. It does show a linear increase in velocity though. All I can say is be careful.
If that is the case and 55.7 blew a primer, I would give myself more of a buffer than 1 grain and 40 fps from blown primers. A hot day, ammo sitting in the sun in your truck, a warm/hot barrel, or many other factors can easily spike 40 fps or more. Personally, I would explore the lower node, just under 3300 fps. 2+ grains and 100+ fps is a little happier saftey margin from blown primers for me personally. Plus your expensive brass will last longer.