On my ea6530v3, cherry-picking this commit: https://github.com/openwrt/openwrt/commit/4b37e3bc2b2a079c996b6d97b8d3dbbd4ba6eb62 into openwrt-21.02 fixes the issue entirely, but it seems as though it doesn't work for all boards, per @pennyweight's last post. I'm not even sure if I can say it's applicable to all ipq40xx devices, but the patch in this commit is at least a part of the solution.