Distilling a large amount of data down to a single number is inherently a compromise in terms of comparing two different lenses.
For example: One lens may be sharper at the center of the field but not as sharp at the edges and corners. The other lens may be slightly softer at the center of the field, but maintain that sharpness much better towards the edges and corners.
Even with the same lens, as is the case in your question, differences in camera design can affect results.
Some of the things, other than sensor resolution, that can dramatically affect the acutance of images taken with various cameras:
- The presence (or not) and strength of the low pass filter in front of the sensor. These are sometimes called anti-aliasing or moiré reduction filters. The Canon 7D, for instance, was known to have a strong low pass filter compared to the 50D that preceded it and the 7D Mark II that followed it. Images from the 7D, even those made in controlled laboratory conditions, suffered from the stronger filter. On the other hand, video footage from the 7D (which is inherently lower resolution that 20MP still images by a factor of 10) looked better with less moiré. The 1D X Mark III has a different type of newly developed Low pass filter. Compared to the 1D X Mark II, which had the same basic sensor with a different filter stack in front of it, the Mark III performed noticeably better. The 45MP EOS R5 with the same kind of newer technology low pass filter beats the older 50MP EOS 5Ds with the old style low pass filter.
- The thickness(es) and refractive index(es) of the cover glass(es) in front of the filter. The cover glass may include coatings for infrared and/or UV cut filters, or these filters may be separate ones in the stack in front of the sensor.
- The design and manufacturing quality of the microlenses in front of the Bayer mask.
- The properties of the Bayer mask itself.
- Processing and amplification of the analog signal from the sensor, the bit depth of analog to digital conversion, and the processing routines applied to that digital information.
- Even two copies of the exact same camera model can test with slightly different results due to the variations caused by manufacturing tolerances, especially with regard to the distance and squareness of the sensor with the camera's lens mount ring.
If you want to get the most acutance out of any lens that projects an image circle large enough for a full frame camera, then use a full frame sensor. The reason for doing so is the difference in enlargement ratios between a full frame sensor and an APS-C sensor when viewing images from each at the same display size.
These differences between enlargement ratios due to sensor size are significantly greater than the minor differences between different cameras with the same sensor size.
Why is FF sharper than crop body for the same framing of the same object?
When you enlarge the image by 1.6X more linearly, which is 2.56X more by area, you also enlarge all of the blur in the image from the smaller sensor that much more by the same factor. Some of the blur that can't be perceived as blurry by the viewer when enlarged at the lower enlargement ratio required to view a FF image at a certain size will be perceived as blur at the higher enlargement ratio required to view an APS-C image at the same display size.
To get the same acutance from an image enlarged by 1.6X more, you need a lens that can resolve 1.6X more lines per millimeter.