Thanks for doing the test.
I did the same test; set a scheduled sale while running the Twenty Twenty-One theme and WooCommerce. On my side, the start date of the sale did not disappear when the start date of the sale was reached.
Thus, from the info at hand and the tests you’ve done, the cause is not apparent. Further, the issue cannot be replicated on a test site. Therefore, you will want someone with access to your site to take a closer look at what could be causing it.
Here are some ways to proceed:
- Engage your host to check the issue further, there might be issues on their side like some issues with cron jobs
- Consult with a developer to look keenly into the issue.
- If you have any purchases from the WooCommerce marketplace, you can create a ticket with us.
Thanks and all the best.