Sadly this is not the first instance I have heard of local to me also. I have rehomed several cats quite happily where there is no cat flap - one of my little kittens Lunar is going to a new home this week without one. They are fitting one next year, when she is spayed and old enough to go out. Some of these cats have gone to loving homes who have been turned down by other rescues for this reason.
I personally (and this is only my opinion) prefer there NOT to be a flap initally as there is less chance for a cat to gain access to the outside in the first few weeks.
Mine is on "in-only" from tea-time, but my little Lily can still hook it open!
Frustratingly she has also kindly taught Polly - shy CovCats foster - to do the same in the last few weeks
so I have now 2 nauty-tortie-white girls to try to get and keep in each night. Get one in, then try to get other before 1st escapes again. Madness at times! Must be something in the colour