A DESCRIPTION OF THE PROBLEM :
The documentation for Matcher#usePattern only says
>This method causes this matcher to lose information about the groups of the last match that occurred. The matcher's position in the input is maintained and its last append position is unaffected.

It should probably also mention that hitEnd() and requireEnd() remain unchanged as well (e.g. remain true). This might not be that obvious since both methods are usually used in combination with the current pattern and you might expect that their results are reset as well when the pattern is changed.