Question:
Are you using that snippet by itself? If so, Pandoc might not be able to figure out it's a slide. You need the level 2 heading for a slide title at the very least, and the frontmatter helps too. Like this:
% Pandoc for Powerpoint
% Peter Conrad
% 1 March 2021
# Section Title
## Slide Title
Text:
* Bullet
* Bullet
::: notes
Speaker notes go here
:::
## Two-column slide with image
:::::::::::::: {.columns}
::: {.column width="50%"}
Left column:
* Bullet
* Bullet
:::
::: {.column width="50%"}

:::
::::::::::::::
::: notes
Speaker notes go here
:::