The situation here is that because half of the field was involved, a recall gun should have been fired after 100m.
The officials blew it and the results shall stand. As people have mentioned, just like an official missing a call in football or any other sport.
Over 25 laps of tactical running the impact of the mistake is minimized.
Any records set by runners running less than the full distance would not count.
This is like an athlete winning a long jump competition on a wind-aided jump while second place had wind legal jumps.
The best mark wins but no record can be set.
It's not fair but it is as fair as you can get under the circumstances.