I don't know since, but I think in previous versions of amanda this wouldn't happen.
When I have a machine that for unknown reason can't be reachable the planner fails for all machines and the backups are aborted for all machines.
FAILURE DUMP SUMMARY:
planner: ERROR Request to x50 failed: EOF on read from x50
x1 disk1 RESULTS MISSING
x2 disk2 RESULTS MISSING
...
x100 disk100 RESULTS MISSING
NOTES:
planner: tapecycle (100) <= runspercycle (100)
driver: WARNING: got empty schedule from planner
The version of amanda is 3.5.4