There are many B450 boards that plenty of users are reporting just don't play nice with R7 3xxx series processors, even though they claim support. There is some pretty big disparity in the quality of these boards such as 3 layer vs 5 layer boards. For many of the boards the 2nd gen Zen chips overheat the VRMs that worked fine with previous generations. Some have luck keeping the core counts lower for instance an R5 will be stable when an R7 isn't.
I have seen more complaints about the Gigabyte boards in general and specifically with memory issues than most other boards in these forums and elsewhere.
I would say that assuming that you have all the right drivers loaded properly you might want to try a fresh install of Windows if you did not already and see if by chance that helps. You could use a spare drive if you have on so you can keep your current drive to revert to if the test makes no difference.
In general Ryzen 3xxx has been more tolerant of a wider spectrum of memory sticks than the prior generation chips were. However an issue can pretty much be anything.
If for instance a memory stick is bad, it may cause more of an issue with one architecture cpu vs another.
There is no doubt that if your memory is not on the QVL list that the board may no set the correct timings. This does not however mean that the memory will not work. It just means there is no auto config for it. You can always talk to the support department for who made you Ram and ask them what timings you should use and enter those values manually to see if that does the trick.
You can try to rule out if a memory stick is bad by putting in only one at a time and seeing if one is stable vs the other.