Even here he is talking about it.
https://www.35mmc.com/04/05/2021/plustek-opticfilm-135i-35mm-film-scanner/
"In case the true plane of focus was slightly above the negative, I tried a shim to raise it. My attempts did not seem to improve things, if anything quality dropped slightly.
It might be that the point of ideal focus is actually below the tray on my copy. Without carrying out some potentially harmful modifications to what is a very fine negative holder I can’t say for certain. Manufacturing tolerances could put the ideal focus point above or below the tray on other examples of the 135i. Others
might get improvements to their focus through shims.
Plustek may have designed the optics in the Opticfilm 135i to maximize depth of focus and avoid the complexity of AF. The downside of that might be a general
slight softness that is not improved by shimming."
Again I tried plustek for slides and old curled film it was worthless.
From personal experience I rather be in control of focus.
That goes for all scanner with fixed focus.
You do whatever you want.