Challenges allow logging after they close

How did you know it was backdated? Someone uploading their activity hours or days after it was done is not uncommon.

How does that allow for a fair challenge then?

I understand someone uploading an activity hours or days after it is done, but it should be within the challenge start and end times.

It seems to me that this person was disappointed at losing the challenge, and created another entry to put themself in the lead, claiming that they did the activity the day before.

I know it was backdated because it says the activity was done the day before. I can see a notification for the new entry at 12:47am, whereas the challenge ended at 12:00am.

A "seems to be" isn't really a proof so I would say in doubt that activity was done in the legit time frame. 

Oh I have screenshots as my proof and sent it to challenge administrators. I shouldn't have to do that though if the Strava app just worked better for challenges.

If the activity was done in the legit time frame, okay, but it should also be input during the legit challenge timeframe. It shouldn't count if it gets put in later. Had it been input within the challenge, it would have motivated me or other participants to go further and push harder so that we could surpass them. Putting it in after the challenge deadline doesn't give us that opportunity.  They should have had it logged before the challenge deadline, and Strava shouldn't allow logging after a challenge is set to end. Would it be fair if I went and logged something 2 weeks after the challenge closed? No. 2 hours, 2 weeks, both are beyond the deadline.

Are you sure its not because they started the activity before and finished and uploaded at like 1 am?

Usually the challenge specification says explicitely how long after the end of the challenge we are allowed to upload activities for it. Mostly that is 3 days. Your objection regarding the motivation if you don't know about the latecomers is of course justified. But it would be a bit cruel to make immediate uploads mandatory, not everyone can do that technically or because of the infrastructure.

I can't speak for that specific activity, but I do know that I have had a number of activities over the years that failed to upload/sync when I completed them.  Often, unless I go check specifically for them, I won't realize it until I do another activity and the previous one that didn't upload finally loads.  Sometimes it is because I turn off my device before it gets a chance to sync so it doesn't complete that process until it is turned back on and has a connection, which could be hours or a day or more later.  I totally understand that it feels like you were cheated, but it doesn't mean someone is intentionally trying to cheat.  Would it be fair to the other person in the challenge if they did all of the work to win the challenge, yet lost because they did not have cell service where they were at to upload their activity?

It doesn't sound like they cheated and did it in the rules. You are allowed some time after a challenge ends to upload the related activities, it would be unfair to not allow that for all the reasons everyone else has said. Timestamp will be from GPS, not impossible to fake but you'd have to be determined.

If someone input a manual activity then you may have cause for suspicion but thats for the challange admins.