Crop wont uncrop to full frame

One thing to check, is whether you have a preset assigned as default processing, which includes transformation of the sorts mentioned.

Reset does not, strictly speaking, "remove all adjustments". It (re)applies whatever are the current processing defaults for that type of image. If those defaults are ever changed, the result from using Reset changes correspondingly. 

Note that depending on what was applied, if both Upright and Transform settings were changed you may have to reset each of those options groups separately.

So in total, collecting what has been suggested so far, reset as needed:

Crop tool

Upright

Transform

Lens Correction

Check that you don't have some Transformation/Geometry settings being applied to the image. Lens Corrections can also crop the image a bit.