We just wasted several hours trying to deploy an EdgeRouter X SFP using the SFP port. The problem is pretty easy to reproduce. Using a FiberPoE and and EdgeRouter X SFP, connect the two devices using a couple of SFPs and fiber, a SFP direct attach cable (DAC), or a 1000baseT copper gigabit SFP. If the SFPs are plugged in when the EdgeRouter X SFP is first powered up, the link is successfully negotiated, and the FiberPoE lights up blue indicating the presense of link on the fibre. If you then remove link from the SFP (unplug the fiber for a few seconds, then plug it back in), the EdgeRouter X SFP will no longer negotiate link on the SFP port. Instead, the FiberPoE sits there with its indicator light being white and its copper port down. Power cycle the EdgeRouter X SFP, and the link will come back up until the fiber link is flapped again.
This behaviour makes the EdgeRouter X SFP unusable with a fiber connection. Any time the link drops, it won't autonegotiate correctly again. Maybe, if the EdgeRouter X SFP could be forced to gigabit speeds this wouldn't be an issue. Using a pair of FiberPoE in the same configuration works perfectly.
This behaviour is the same on firmware versions 1.8.5 and 1.9.0.