Can you validate than when you load on the same system that the default browser on your computer honors the end time? I can’t recall if PowerPoint will use it’s own browser for this, the default browser, or IE/Edge engine
Loading the link here it ends on the time stated.
I’m also wondering if using an iframe is part of the problem, as invisible frames can be wonky due to framebusting code and browser handling intended to deter invisible elements from trying to evade security measures.