Are any scheduled workflows working for you in your environment?

Yes, others are working - the overnight ones are not working

Perhaps it is being interfered by a Microsoft administrative tasks.

For example Microsoft will make a backup every 12 hours on your tenant.

If you can, try to browse your site at that time, and see if it is online.

Another test you can make is to delay a workflow to 1AM and then loop every 5 minutes and loging workflow history messages, to realize what 's happening on that time.