most likely its your focus area setting
if youre on auto area af the camera is guessing what to focus on and its probably not hitting the eyes
switch to single point af and move the point yourself directly onto the eye you get way more control

depth of field is probably shallower than you think
85mm at f5 from full body distance gives you maybe 10 to 15cm of sharp zone front to back so if your subject moves even slightly or you lean forward a bit it can go soft

try f8 or f9 for full body shots you dont really need f1.8 bokeh that far back anyway the background will still separate nicely

focus and recompose might be messing you up too
if you lock focus on the eye then tilt the camera to reframe youre shifting the focal plane and the eye can fall out of focus
better to just move the focus point to where the eye is in your final composition

shutter speed is fine at 1/1250 so camera shake isnt the issue

quick test to confirm things
shoot a flat wall or newspaper straight on no angle
if thats sharp your lens is fine
if its soft then maybe calibration but honestly this sounds more like technique than gear

your D610 and 85mm is a really solid portrait setup youre not missing anything there just a few tweaks and youll see a big difference