Echo double-encodes URI 'fragment' part when removing the 'markasread' parameter. This happens in all browsers I tested.
For example, visit this URL (taken from a notification email I got): https://pl.wikipedia.org/wiki/Wikipedia:Pro%C5%9Bby_do_administrator%C3%B3w?markasread=10680642#Wulgaryzmy_z_b%C5%82%C4%99dami_ortograficznymi_do_filtru_nadu%C5%BCy%C4%87
After the 'markasread' parameter is removed, it changes to: https://pl.wikipedia.org/wiki/Wikipedia:Prośby_do_administratorów#Wulgaryzmy_z_b%25C5%2582%25C4%2599dami_ortograficznymi_do_filtru_nadu%25C5%25BCy%25C4%2587
The correct URL would be: https://pl.wikipedia.org/wiki/Wikipedia:Prośby_do_administratorów#Wulgaryzmy_z_b%C5%82%C4%99dami_ortograficznymi_do_filtru_nadu%C5%BCy%C4%87