TBH most “false positives” that I investigate are wishful thinking or the result of ignorance of what is really happening. It looks like you are using Debian. That probably doesn’t help. Here is a typical Debian “bug” report:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=802778
10 years old. It never was a false positive. It was fixed a good few years ago. The fix did not involve suppressing the error.
Valgrind does need a lot of work, especially for missing CPU features and for Darwin. I’m not aware of many memcheck bugs that aren’t relatively obscure corner cases.
If you have encountered bugs please report them to https://bugs.kde.org.
Like the last one was/is the inability to comprehend safety of large buffers on ppc64el because the stack clash protector code generated by gcc isn't understood. The one before that was more problems of that sort on armhf where it also didn't understand the clash protector - in more cases.
It's quite surprising and it takes days to weeks to debug each of these, going down to the assembler level and verifying that by hand.