patching veneer sheet grain/knot holes

When purchasing 4 x 8 veneer sheets in a knotty type finish (alder) are the knots usually filled in by the manufacturer...or is this normally done by the end user ? I purchased a sample sheet from Woodworkers Source...and most of the rough knots appeared to be already "puttied" or "filled" with some filler and then sanded along with the rest of the sheets. I just unwrapped 1/2 dozen sheets from another local supplier ( American Hardwoods, Phoenix) and all the sheets have no patching or filling in of the bad knots. You can see the paper backing and the light shadow if you hold them up to the light and look from the paper side ?
Is this normal...or did a get 1/2 dozen rejects from the yard. If normal and I need to "patch" the knot holes where the paper shows through...what do I use to match the alder and "fill in" the knots ?
Thanks, Tim
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