The images you've posted are a classic for a missing I-frame in long GOP (also called IPB) footage. Every so often there's an I frame, which is the complete picture, and then a set of P and B frames, which are differences. If the I frame is lost, the differences are applied to a blank frame; your first two pics look exactly like that. In other words, this looks like a corrupted file.
You said that this issue is in the ORIGINAL files direct from the camera; you need to be sure of that, because this is crucial to knowing where the fault is. Also it would help if you could tells us what exact video mode you were using.
With all that said, I'm inclined to suspect your SD card. You need to be sure it's fast enough. The most important thing to know is that the speed rating printed on the card (what you quote as "150mb/s") is complete garbage. Also it's important to know the difference between lowercase "b" and uppercase "B". I've made some videos which explain these issues: https://moonblink.info/MudLake/tech
Bottom line, the only thing on the card that matters is the "v" speed rating, which is v30 in this case. That means the card can take video at 30 MB/s (MegaBytes per second), which is 240 Mb/s (Megabits per second). However the R6 can record long GOP video at 340 Mb/s, according to the specs. In all-Intra, it goes up to 470 Mb/s, which is 58 MB/s, so I would recommend at least a v60 memory card.
Having said that, if you're recording at a lower rate -- like 4K @ 29.97 FPS -- then the data rate should be 170 Mb/s, or 22 MB/s, which *should* work with a v30 card.
Whether this is your issue I don't know, but it seems like a place to start.
