I do not have direct experience with this but here is a link that has some information that might help:
JaLa Yorkshire Terriers...Luxating Patellas I have read here on YT that many puppies have loose knees but tighten as they mature. Thankfully mine have all had tight knees at their vet exam. I understand it would be difficult to think that a few weeks later they had Grade 2 LP and that it would be genetic, especially if it was just the one knee. I would be thinking as you are that it might have been due to the jumping. Is this a pup you have sold? Are you trying to decide what to do about warranty or future breedings?
If none of the puppies resulting from breeding that dam and sire, nor the dam and sire themselves or their parents had ever had history of LP and you had just recently had them checked as free from LP, I would really be thinking injury instead of genetic. But there is probably nothing to cover at this point anyway. Early grade 2 is usually not operated on, especially in a 5 month old. So it is one of those wait and watch situations. I would still offer to take the puppy back if anyone was unhappy about it.
If this is the only time you have seen this, I would not discontinue the breeding. It is not really pointing enough to anything genetic to tell you to stop using this dam and sire together or to stop breeding either of them. But this is jmo and I would get the opinion of one or two of the more experience breeders. If nothing else, my post will give your post a bump and maybe you will get more input!