Many authors write stories with twists. For a while, Law and Order even advertised the fact that there was a twist at the end of their episodes.
The problem with twist endings and flash is that they often don’t feel like twists: they feel like punch lines. And that makes the stories jokes, not stories.
It makes sense. In a longer story, you have a lot of space to build up people’s motivations, desires, personalities — characters, in other words. When there’s a twist, you’re seeing the twist as a change in a well-established character’s knowledge. In flash, none of the characters are as strongly developed, so the twist is often more of a change in the reader’s knowledge. And that’s just a punch line.
Sometimes the twist is even worse: the author withholds information from the reader even though the main character would definitely know it. Sometimes that happens in longer works, but I think the temptation is stronger in flash because it would have to be sustained for a shorter, and therefore seemingly more manageable, length of time. But it’s still a case of the author cheating the reader.
Writing a story develops trust between the author and the reader. Withholding information from the reader breaks that trust; making a joke of the story makes the reader take you a little bit less seriously. Unless you’re deliberately trying to not be taken seriously — and I’ll submit that most of the best humor is very seriously humorous — I’d avoid both.