Figure alignment different with/without caption

[austin-hoover]

index.md:

Some text.

![](image.png){width=150px}

![This is a figure caption.](image.png){width=150px}

Running pandoc -o index.html index.md gives

Is this expected behavior?

[jpcirrus]

Yes. The description in the manual for the pandoc markdown extension implicit_figures states:

An image with nonempty alt text, occurring by itself in a paragraph, will
be rendered as a figure with a caption.

Your first example is just an image in a paragraph, not a figure. To turn it into a figure, and so centered, you can use a non-printing character, for example a non-breaking space, as a "caption":

![ ](image.png){width=150px}