Idiom for two people having two drastically different appraisals of an event

Solution 1:

This sounds like an example of the Rashomon effect. From Wikipedia (footnotes omitted):

The Rashomon effect occurs when an event is given contradictory interpretations by the individuals involved. The effect is named after Akira Kurosawa's 1950 film Rashomon, in which a murder is described in four contradictory ways by four witnesses. The term addresses the motives, mechanism and occurrences of the reporting on the circumstance and addresses contested interpretations of events, the existence of disagreements regarding the evidence of events and subjectivity versus objectivity in human perception, memory and reporting.

So for example if A and B go on a date, A might remember that the food was terrific and they were incredibly witty, but then B got increasingly tipsy and there was a weird digression into French art criticism that tanked the whole date; while B remembers that the food violated their dietary restrictions and A kept telling total groaners, but the wine was terrific and there was a fascinating conversation about French art criticism that really made them want to see B again. And the waiter might primarily remember that A and B totally stiffed him on the tip in spite of ordering three appetizers, two desserts, and three bottles of wine.

To address the request for an idiom: The term Rashomon, applied as an adjective to various nouns, has come to be an idiom¹ for this kind of encounter. For example (bolding added; other emphasis original):

This turned out to be what I call a Rashomon date, where the perspectives on how it went differ wildly.
—Kashmir Hill, "My Weird Hobby: Matchmaking Lawyers", Forbes, February 14, 2011

Watergate's Rashomon moment: Fans of All the President's Men . . . may have wondered why the anonymous source then known as Deep Throat didn't simply reveal everything he knew. In telling the story from the perspective of Deep Throat . . . writer-director Peter Landesman helps us understand the G-man's justified paranoia. The film is a thriller, but the current occupant of the White House may find it to be a horror film . . . .
—Simon Houpt, "The TIFF movies we can’t wait to see, from a tiny Matt Damon to two tennis tales", The Globe and Mail, September 8, 2017 (describing the film Mark Felt: The Man Who Brought Down the White House)

Let’s face it: We live in a Rashomon reality in which every event is instantly captured from a dozen angles and given at least as many interpretations . . . .
—Antonio García Martínez, "Journalism isn't dying. It's returning to its roots", Wired, 02.10.19

Perhaps unsurprisingly, the use has picked up quite a bit in US news reports in the past few years.

¹_{I am taking "idiom" here to mean "an expression whose meaning is not predictable from the usual meanings of its constituent elements" (Dictionary.com), since the usual meaning of something like "Rashomon encounter" would be "an encounter with Rashomon".}

Solution 2:

one man's meat is another man's poison (TFD)
That is, there is no accounting for tastes.

What one person may consider good, enjoyable, or beneficial may be disliked by someone else.
A: "I really don't understand the appeal of the Harry Potter series. There are so many better books out there!"
B: "Eh, one man's meat is another man's poison."
(Farlex Partner Idioms Dictionary © Farlex 2017)

See also:

You may not like something that I like. The phrase, which was first written by the Roman poet Lucretius, was appropriated to refer to any situation where two people disagree over something. The 20th-century literary wit George S. Kauffman's most celebrated pun was “One man's Mede is another man's Persian.”
(Endangered Phrases by Steven D. Price Copyright © 2011 by Steven D. Price)

Solution 3:

I couldn't find anything specific to an 'event' experience, but perhaps you could use:

Different sides of the same coin

Two things that appear different, but have a connection/are related