Text Editor truncates posts when I click Save Draft

I compose blog posts in HTML using Sublime Text and copy-paste the content from there into the Text view in the WYSIWYG. For several months now, the following happens:

1. I paste the HTML in the editor.
2. I click “Save Draft”.
3. The editor saves only a portion of what I pasted in; it truncates the post at a random point.
4. I try again, repeating the above steps.
5. Sometimes, after two or three tries, Save Draft saves the entire thing.
6. But if I edit the post in the editor (even change the post title), Save Draft will again truncate the post at a random point.
7. If I manage to save the entire thing, clicking “Preview” will show a truncated version of the post.
8. Sometimes clicking Preview will open the editor in a new tab with the post… you guessed it: cut off at a random point!
9. This happens in Safari and Chrome. I’m running Mac OS Sierra 10.12, not that that should matter…

Today WordPress is completely unusable due to the above. I have relied on the compose-in-Sublime and paste-into-WP workflow for several years now.

We’re using 4.9.4; I think the problem started with 4.9.2 or 4.9.3.