Document does not contain an HTML syntax then it will be converted to Markdown and there’s no Markdown syntax to center text, images, titles etc. To center texts you need to use HTML, as example:
<div align=center>Centered text</div>
will become
This is not a bug and is happening because you added a dotted line immediately under “Strong Text”
See as example:
Strong text
---
will be transformed in H2 heading:
Strong text
More info at babelmark3 | Compare Markdown Implementations.
In markdown a dotted line will be converted in a horizontal line if it is separated from the text with a blank space:
Strong text
---
will be transformed in:
Strong text
You can use the CommonMark tutorial to learn how to use the Markdown syntax if you need it.
You can also try this theme component, MD Composer extras - theme - Discourse Meta which adds extra buttons to the composer, including those for alignment.