Prepending text in a GB image block breaks GB editing

[hypest]

Expected

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

Observed

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

Reproduced

1. Load the following html snippet in html mode:

<!-- wp:image {"align":"center"} -->
<figure class="wp-block-image aligncenter">
  <img src="https://cldup.com/cXyG__fTLN.jpg" alt="Beautiful landscape" />
  <figcaption>If your theme supports it, you'll see the "wide" button on the image toolbar. Give it a try.
  </figcaption>
</figure>
<!-- /wp:image -->

2. Switch to visual mode and place the cursor in front of the image
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 <figure> 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

[koke]

Tested this on iOS and Classic editor and they both insert the text between figure and img