That's odd. o.O I've been using RTSS to cap the frame rate and recording with OBS without too much hassle.I took a day to figure it out, but it appears OBSm cannot hook properly when RivaTuner Statistics Server is hooking to the game as well. It's a tool to display a framerate counter as well as capping the framerate (Fallout 4 requires this.)
I've always had a Global profile in RTSS set to Detection: None, but I guess that wasn't enough.1. Add OBS-MP to the list in RTSS - Set the detection level to "none"