If you're taking deer with the Speer 400gr FP in the 1800fps range, and you haven't had one (or more) come apart, consider yourself lucky.
Its not a constant at that speed, but it can happen. Deer can still be DRT, and usually is, but that bullet isn't built for that speed. And I can tell you from personal experience that if you jack the speed up to 2100fps, (.458 Win) they act like very LARGE varmint bullets!
That bullet is built to give expansion at the low end of .45-70 speeds (black powder speed ~1200fps), and they will work well driven a bit faster, but from what I've seen, 1800fps is the edge of where they start coming apart.
There are lots of other bullets that are built to take that kind of velocity (the Hornady 350gr RN is an excellent one), just not that Speer 400 FP.
Hardcast 400gr bullets at 1800 perform very well, don't shed bits of jacket into the meat, and will eventually stop a couple feet deep in the first tree thick enough that they run into after going through the deer.
The Speer 400gr is a great bullet, excellent for a lot of things, but you CAN drive them too fast in modern guns with heavy loads.