In a recent Community platform update from Insided, the way URL's are constructed has changed. In a reaction Insided said the following: 

The URL’s of ideas were inconsistent to other content types, so we changed them to be consistent (this was required for conversions to work).

If a user has copied an idea URL before April 19, then this URL is not going to work anymore. The URLs are not going to change in the future, so whenever a user copies an idea link from the Community now, they will always see the correct topic.

We apologise for the inconvenience