Unless it has been modified, it would be worth searching Bootstrap issues for this problem, otherwise it needs to be reported to them. It's a serious issue, but might be a custom style of ours breaking it.
There is no real mention of this issue anywhere from what I could find. However I did find a comment indicating that Bootstrap 3 does not officially support the default Android browser (not sure about BS4)
It appears that the navbar expands both up and down, with the third field (curriculum guides) being positioned in the center.
By setting $navbar-height (in _core-variables.scss) to 10rem you can see that the dropdown does indeed center itself in relation to the button (and at this height 'chapters' can be clicked on, but it looks silly)
Here is a code snippet approximating the state of the website without all the fluff (the ul & li tags instead of div and a tags make no difference).
The issue does not manifest with this example, indicating that it is an issue on our side, rather than with Bootstrap