I'd consider the old one as the same bug; however, its exact side effects depend on many factors (in particular, how things are aligned in memory being one important factor). That probably explains why, with the build and the combination of settings you used, the probability of getting an invalid MLV (with the same type of error) was pretty high (about 10% from your report), while nobody else reported the exact same issue (so, either the issue occurred in places that did not affect the MLV structure, or, if it was, either it wasn't reported - likely if it happened once in 1000 clips or so, or, if it was reported, we had no idea what it could have been).
These "random" errors are usually very hard to track down, but in your case, the only hypothesis that made sense (how it could happen) was that CPU was aware of the size of the NULL block (and computed it correctly), but for some reason, what was written to card (which is performed using DMA from physical memory) was wrong. Therefore, there must have been some sort of mismatch between what CPU thinks about the physical memory, and what the memory actually contains (and that's likely because the has only written some changes - the contents of that NULL block - into its own cache, but these changes did not reach the physical memory by the time the NULL block was written to card).
I knew about this quirk, and most of the code doing file I/O uses memory buffers with caching disabled (fio_malloc), but since this wasn't enforced, these calls (those for writing the MLV headers, which were used incorrectly for many years) didn't ring a bell. Now, the PR either enforces correct usage (when reading files) or just ensures everything is written to main memory before writing files (so there will be no more restrictions about what memory addresses can be used for writing).
And yes, similar issues can be in other parts of the source code (although I didn't find any by looking at the code), or in code written by new developers, who may not be aware of this quirk (as there were no warnings, and so far, on the other platforms we were familiar with, this was a non-issue - afaik, the cache coherence issues, if any, are handled by the operating system). If there are such problems in other areas, after the fix, you'll get an error text all over the screen.