I updated /lib/firmware/amd-ucode/*.bin from upstream git (commit 7bc246451318b3536d9bfd3c4e46d541a9831b33) and updated initramfs (I'm not sure that's the right place) and since then I haven't had the problem anymore. I realize that we're in freeze mode, but the 18.50 release looks to become important for proper functioning hardware pretty quickly. CC-ing Kernel Team to make sure they're aware too.
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