I don’t have more experience but I agree with your intuition. I would suspect the seed ripening rate to be governed by the mother plant characteristics. However ripeness uniformity across the kernels would also be a characteristic of the mother plant. And I would suspect a mother plant resulting from a cross could introduce a change in that uniformity. I’ve heard other things like not to grow water melon near cucumbers because they will cross and your watermelon will taste like cucumbers. Clearly that is relevant for land races, and I suspect the wisdom is old from a time where more people were practicing saving seed. But atleast how that advice was passed to me ( that it will change the flavor of this years crop) I think is actually false, again the fruit properties should follow the mother plant. Because the genetics of the cross are in the seeds not in the fruit. The fruit is the flesh of the mother plant. This is how apples are.