Try embedding your video like this:
<video controls>
<source src="https://www.domain.com/wp-content/uploads/2018/07/5898ca6e5276e4cd97c0bd730a2ed82c11780595-480p__80044.mp4#t=18" type="video/mp4" />
</video>
Paste this in your editor but make sure on the top right it is set to “Text” not “Visual”. When you switch it back to “Visual” you will just see blank gray frame. Preview the post and you should see the video embedded with the start time.
Note the controls tag, if you remove that then the video won’t load and it will just show you a screenshot. So what you are doing is essentially writing your own html embed code.
You can also add other tags to the video such as width / height, autoplay, etc.
Hope this helped!