My thought is that it may have something to do with the white plastic jewel case trays, but I'm only guessing here.
If the discs were the same ATIP then it could be explained as a design defect. And two different ATIPs from the same factory could be a manufacturing defect. But this...
Up until a few weeks ago, the only problematic discs that I had come across were made by TY in 1998. Only recently have I come across problematic TDK discs also made in 1998.
Has a problem been observed with any similar discs with a black tray?
I don't think these products ever shipped with black jewel case trays. All batches that I have seen shipped with white trays.
Can you tell if the growth is on the surface of the disc or inside? (You can try cleaning part of the surface with isopropyl alcohol.) And do the paper inserts, disc or case interior smell musty/mouldy at all?
The 'growth' is on the surface of the disc and only exists on the recording side. It can be cleaned off with isopropyl alcohol but a thorough clean requires at least three passes using separate clean microfibre cloths. When opened, each case smells 'as new' with no undue bad odours.
If the growth is on the surface then I would suspect that there isn't a inherent problem with the discs. Most likely either the fungal contamination was carried by the inserts or the cases, although it could have penetrated the cellophane wrap if it wasn't completely sealed (as is frequently the case). But if there is a fungus attacking the layers inside the disc structure, that would be far more worrying - it may have spread from outside, or the original source may have been seeded inside the disc.
I hope that it isn't the early signs of a more general problem which may afflict all early CD-R.
What's really interesting is that the Sony CDQ-74BN/CDQ-74CN products are the only ones that I have come across that suffer from this issue. I have other products that date back to the same production year that were made by the same manufacturers and even distributed in the same countries (i.e. in North America) that show no signs of this defect.
I wouldn't worry about older CD-Rs unless they are Sony-branded and from 1998.