During the nodetool operation of replace node, the replacing node is reported with a status of "UN" when it is still not really normal but rather in initializing state. it should report a different status like "UJ" for example.
see example in this comment:
scylladb/scylladb#9629 (comment)
The original scylla issues with details are:
scylladb/scylladb#9629 (comment)
and:
scylladb/scylladb#1793