Describe the bug

If you delete at least one character from the start of a paragraph block, then continuing to hit backspace will not merge the block with the previous block like it is supposed to.

To Reproduce
Steps to reproduce the behavior:

1. Add a pararaph block to an article and add some text to it
2. Click in the middle of the text and hit enter to move the right half of the text to a new block
3. Hit backspace and watch the new block merge back correctly with the previous block
4. Hit enter from the middle of the text again to create a new block
5. This time delete one or more characters from the beginning of the new text block
6. Continue to hit the backspace key => note that the block is no longer merging with the previous block like it was in step 3.

Expected behavior
Pressing Backspace from the beginning of a paragraph block should always merge it with an existing preceding paragraph block, regardless if you deleted characters at the beginning first.

Screenshots

Smartphone (please complete the following information):

• Device: Galaxy Tab S3
• OS: Android 10 (API 29)
• Version Gutenberg-Mobile release/1.22.0