Hello Andreas,
By default, JUNOS uses round-robin to balance authentication attempts between all configured RADIUS servers. When a server doesn't respond, it's marked as unreachable. After expiration of revert timer this server is marked as UP again (without any actual check for "liveness" of the server taking place), and this process goes on and on. The following KB describes the revert-timer configuration option:
https://kb.juniper.net/InfoCenter/index?page=content&id=KB15858
The best solution here is to remove dead servers from configuration, which you did already.
HTH
Best regards,
Sergii
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