Me too, although not nearly as large a drop as you're experiencing. My RHR averages around 58, but it'll go down to 54 or lower when I'm in PEM.
Occasionally, with more severe PEM, my heart rate will drop into the low 40s whilst I'm upright, which is horrible – my heart's straining and I feel as if I'm going to pass out. It's a rare event, as I try not to push myself into that state, but if something's gone wrong it's not always possible to avoid it.