The xmlui browsing pages are publishing a hidden field for "starts with" filter that breaks the "starts with" form.
To reproduce this error:
1) Go to browse by title: http://demo.dspace.org/xmlui/browse?type=title
2) Filter by some text (by example: document)
3) Filter again by some other text (by example: test)
Result: With Mirage2 theme, the browsing page goes to browsing by date because it lost the "type" parameter.
With Mirage theme, the user-submitted value "starts with" is ignored on the second search.
I think it's related with this commit: https://github.com/DSpace/DSpace/commit/7b2050e1de24a79ae6fa4412e5bb3bd9e3cea8a9
and therefore it only affects to 6.3 version.