Prepending text in a GB separator block breaks GB editing

[hypest]

Expected

Adding text in front of the horizontal separator in a GB separator block should not break the GB editing experience (This block appears to have been modified externally. placeholder).

Observed

Adding text in front of the horizontal separator in a GB separator block causes the This block appears to have been modified externally. placeholder to appear in GB, breaking the editing flow.

Reproduced

1. Load the following html snippet in html mode:

<!-- wp:separator -->
<hr class="wp-block-separator" />
<!-- /wp:separator -->

2. Switch to visual mode and place the cursor at the beginning of the horizontal separator
3. Insert one or more characters
4. Switch to html mode and notice that the character(s) entered are inbetween the starting GB comment and the <hr> element. That is not valid for GB and causes the "modified externally" placeholder to appear.

Tested

Nexus 5X on Android v7.1.2, Aztec v1.3.6