Copying from https://meta.wikimedia.org/w/index.php?oldid=3681360#Issues_clicking_the_.22discussion.22_link:

If I recall correctly, the hiding-method was changed in 1.18 or 1.19 because display: none; had side-effects defeating its very purpose! Screenreaders are given a link that jumps to #jump-to-nav. However browsers can't scroll to a section that has no place in the rendering, it has to have a place in the document structure and also in the visual rendering (so that the browser can calculate to which pixel it should scroll). The element can be transparant and only 1x1px in size, but it must not be display: none;. This new hiding method was apparently only tested in Vector and works there, but it needs fixing in Monobook (or perhaps a different hiding method that doesn't cause this problem in Monobook). Either way, using display: none; (as you did now) is not an acceptable solution in the long term. It's okay for now since it is an understandable decision to prioritize the ability for most people to click the navigation links at all, then to allow the smaller part of the population to be able to skip to the navigation using screenreaders, but really both are important groups so this should be fixed soon and this local change reverted.

This bug may be a duplicate of T36893.