Another idea: AFAICS there have been a few memory-bw zen-related bugs reported to likwid and, in particular, https://github.com/RRZE-HPC/likwid/issues/535 may suggest that you could be hitting a similar bug but with another CPU series.

The bug report used AMDuProf to confirm that the bandwidth is actually ~2x than what likwid reported. You could try the same.