Lowkey starting to think that's the standard these days, give a bunch of shit at the beginning then slowly decrease pages as it goes along. Either artist are getting less motivated or just getting lazier.
A lot of first chapters are basically a pitch to an editor/publisher, and tend to only get slightly redone if the series gets approved. That's why so many first chapters can be so long and yet somehow feel like they're progressing at breakneck pacing (especially fantasy/isekai).
Then, once the hooks are in, the mangakas have far less time to actually get chapters out, which often means shortening and/or plot deterioration.
Rather than getting lazier, it's usually a sign of dwindling resources, burnout, late finalized decisions/revisions on the plot, inexperience with themes of the chapter, etc.
I would bet this mangaka is really unsure about how to actually conclude this duel in a way that doesn't feel cliche, or generic, and is also likely generally slower at drawing combat panels.