Site loops indefinitely when sale starts (when cache turned on)

When sale starts the page refreshes … indefinitely.

All products are displayed on the home page (effectively the ‘shop’ page).

When “upcoming sale” countdown timer reaches zero, the home page refreshes — presumably to show the actual sale countdown timer and progress bar — but the page just continually refreshes. I can navigate away to a different page. But then clicking the site logo and returning to the homepage (no params) just puts us back into the refresh loop.

When caching turned off, problem goes away. Site uses very popular WP Fastest Cache with pretty much all extras (eg minifying) turned off.

Any help/ideas much appreciated!