Hi Thanh,
Since it works fine with another site collection, it proves that the workflow has no problem. Also, I’d like to know if the two site collections belong to the same tenant.
Based on the situation, please consider the followings to narrow down the issue:
1. Change the start option of the workflow to have a try
2. Check if there is any other workflow in this list might affect this one
3. Compare the SPMail custom list with the list in the site collection which has no problem to execute the workflow
Moreover, you can also check the workflow history via the list link https://YourSiteCollectionURL/Lists/Workflow%20History/AllItems.aspx.
Thanks,
Nancy Xie