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I don't know exactly how that camera handles back-button AF-C with AF-M -- the X-T2 has a separate "Shutter AF" option for disengaging AF from the shutter release and my X-T10 doesn't -- so I can't help you there. I'd assume, however, that it's normal -- the camera's just waiting for the lens to finish focusing. I personally don't recommend back-button AF-C for general use. If you really need AF-C, then you need it. But if you've got it set to AF-C just in case someday you want AF-C, be aware that if your shooting aperture is at f/9 or higher, the camera will not use phase-detect AF. If you're controlling the aperture, it's usually easy enough to avoid that issue. But if you're shooting P mode, in daylight the camera might select f/9 or higher, leaving you with unnecessarily slow contrast-detect AF. Fujifilm doesn't publish most of this stuff. That's why there are so many third-party books, such as Rico Pfirstinger's that are advertised on this site. Pfirstinger, for example, recommends skipping AF-C for simple tracking AF when you're only going to take one shot. He recommends using AF-S and "mashing" the shutter button down in one stroke, skipping the half-press. That way, as soon as the lens has focused, the shutter is released. That should give AF results as accurate as AF-C. And yes, in single-point AF, the camera will try CDAF if PDAF fails, regardless of which PDAF point you select. Unless you're in AF-C with CH drive, in which case the camera uses only PDAF at apertures of f/8 or below, and only CDAF at apertures of f/9 or above.

That's what Fujifilm's Release Priority does. Fuji will never take a picture while focusing is still in progress -- it always waits for focus to complete. The Release Priority tells it that if the focus completes as a "failure" (!AF would display in the viewfinder) to go ahead and take a picture anyway. With contrast-detect autofocus (CDAF), that's probably a wasted shot, because you don't know where contrast detect will leave the focus when the camera gives up and declares failure. With phase-detect autofocus (PDAF), the lens probably never changed focus, and that's more likely to be usable -- especially during burst shooting. Note that with single-point AF, a PDAF failure will result in the camera switching to CDAF and trying that, so Release Priority rarely will give a usable picture with failed single-point AF. Big exception: single-point AF (on a PDAF point of course) with AF-C and CH and aperture of f/8 or wider will never try CDAF even if PDAF fails -- again, we're talking about a burst shooting setup. If you use single-point AF and sometimes do burst shooting, my personal recommendation is to set Focus Priority for AF-S and Release Priority for AF-C. PDAF failure is close to instantaneous. It sounds like you're experiencing CDAF failure -- either because you're using a configuration that only uses CDAF, or more likely you're using single-point PDAF and the camera is trying CDAF when PDAF fails.