Formula and rollup fields only show text as plain text, so bulleted lists are not available.

However, you can fake a bulleted list by using a text character such as - or * in front of each line.

To do this, I recommend creating the list in a rollup directly. (If you can’t alter the existing rollup, create a new one.)

IF(

  ARRAYJOIN(values),

  "- " & ARRAYJOIN(values, "\n- ")

)

IF(

  ARRAYJOIN(values),

  "- " & ARRAYJOIN(values, "\n- ")

)

To return a 'bullet point' list use the following alternative formula in the rollup.

Thanks for sharing the bullet character. 

I tend to use the - character because it is easier to type on the fly, and I started using it as a bullet several decades ago. Also, the - character is Markdown for a bullet, which is helpful when copying a formula result to a rich text field.