There is been existing malwares like the one called "Bitcoin Stealer" that target and monitor the clipboard to replace bitcoin addresses copied with the hackers addresses,  So this one reconize and replace the address even if it's copied after simple text selection
 The thing you encounter is suggesting that the failure is present on the code of the page, or on the browser

 Ill suggest you if you still encountering the issue to save a copy of the web page, that's trying keep proofs for what is happening