When one of the nodes of a three node cluster rejoins the cluster after some downtime, the PMM dashboard shows a cluster size of one for a short amount of time. Does this mean the donor node is unavailable during SST? After a short amount of time, PMM shows a cluster size of three again.
The graph is using status variable wsrep_cluster_size, which shows number of connected nodes regardless of their state.
Did you check the error logs to see if the donor fell out of the cluster for a moment during SST? What is your SST method?