I don't think you are missing anything. I believe what used to happen in that area was that it defaulted to not put in a time at all when you clicked a date, so when you submitted that it would choose the start of the day since no other time was specified. Now, with it being a drop-down menu that inputs time as soon as you click on the option to add a Lock Date, it's defaulting to the current time rather than not having a time at all. It's probably because it's that drop-down instead of a manual input like before.
For now, the easy fix is to click on the time it defaulted to, then select the 12:00AM option at the top of that list (see attached GIF). It's pretty quick if that's the opening time you need. If you are someone that uses a time different than 12am, then that's where you would need to scroll a bit more or type in at least the start of the time you wish for it to use (see attached JPG).
It may not be as ideal as before, but I don't know if it would be something Canvas would consider a bug to fix. 🤷♀️ I know Blackboard did the same type of thing when our institution used that LMS, so I'm guessing it might be common when the time setting is a drop-down selection tool.