That theoretical diffraction limitation does not really become evident (even in very large prints) until apertures of at least one stop smaller (and sometimes two stops) than those commonly "acknowledged" on most forums, including this one.
To the extent that diffraction might be measured scientifically does not always equate to visible results.
Moreover, I feel that it is the quality of the lens itself, in terms of design, and execution of manufacture (grinding, coating, accuracy of element centering, etc.) plus the overall history and condition of a particular lens, which may or may not leave it vulnerable to displaying actually visible effects of diffraction much sooner than would be noticed in a "better" lens.
In other words (for example) - not all 120mm LF lenses are equal at F/45!
There...I said it...whew!
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