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I got a PCN during an event day as I was parked during the extended period (up to 8:30pm vs the normal 6:30pm). I usually park on this spot as I have dance lessons from 5:30pm on a nearby studio, so I always buy a parking ticket via RingGo app to park from 5:30-6:30pm, and this day was no different.

I was expecting a signage specifying when the next event day is (having parked on streets around the Emirates/Arsenal stadium where bays are marked clearly as Zone G, Zone G2, Zone M, etc.) as per the guidelines on Hackney's website here: https://hackney.gov.uk/matchday-parking

Given the distance from Arsenal stadium (around 1 mile), I never associated this parking spot with football matches, and I expected that the signage would be clear(it does not even specify what zone it is in) if there is an event going on (since it's in such an obscure area). I know there's a number at the bottom of the signage to call a number for events information that I should have bothered to check everytime I parked there.

Do I have grounds to contest this? Or is this a case of, "should have read the signs properly and gave the events information number a call"

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Hackney doesn't put its zone letters on street signs (unlike Islington). But like all such event zones they put next event days on the zone entry signs.
These entry signs control only yellow lines though and some boroughs provide inadeqate parking bay signage (eg Newham) but Hackney's bay signs are much better in signing the times when event days operate and crucially there is the number to call on those small signs although these can go missing or not have been put up.

Arsenal had a home game on 1 April so I think you'll find it hard to challenge this.

They've divided zone M into two and the boundary is Clissold Road, where you parked. So roads east of there were OK after 6.30pm.

You can see the zone entry sign in your last pic. Here's a close up.