It does not waste it, that was also shown on panel.
The extra Haki stayed in the blade and Zoro could call it back.
What we were shown was that Zoro was unaware of how much Haki will be forced out, thus could not make use of it.
A Zoro with Enma mastery can limit how much the blade draws, and then use attacks that expend it because he knows how much he has at his disposal, unlike it was the case with the test run.
The extra Haki stayed in the blade and Zoro could call it back.
What we were shown was that Zoro was unaware of how much Haki will be forced out, thus could not make use of it.
A Zoro with Enma mastery can limit how much the blade draws, and then use attacks that expend it because he knows how much he has at his disposal, unlike it was the case with the test run.