Seen an issue be reported on clusters that have at least 20ish nodes of new members being stuck in the Joining stage. However, those new members are able to join when:

• Another node joins (who then gets stuck)
• The leader gets restarted

It looks to me like the issue, overall, is that there's an edge case where the leader stops responding to some part of the join operation and gets stuck. Been looking into this for a little while but I have more data now and thought to report an issue for other users who've observed this happening.