This is the normal behaviour. Why do you expect anything other?

The respective help topics speaks often about numbers but sometimes also of cells. So, IMHO it's not clear that complete cells - incl. formatting - will be used for filling.

However, I don't see this as a real bug but more as an enhancement - e.g., to make it optional to use more than the cell content. Herefore I'm reopening this issue.

Fill should only fill numbers. You can use Copy to copy formats. I had a column that I remarked cells with colors. When I filled that column, all colors were gone. Help only mentions options for filling numbers.

(In reply to jteera5 from comment #3)
> Fill should only fill numbers. You can use Copy to copy formats. I had a
> column that I remarked cells with colors. When I filled that column, all
> colors were gone. Help only mentions options for filling numbers.

This is your special case. I think most users want the actual behaviour because in the majority of cases equal content should have equal/same formatting especially for filling with Sort Lists.

For your special case there are different solutions. Only 2 as example:
1. copy the cells before filling the row and after that insert only the formats.
2. Use a separate column for marking the row and create a Conditional Formatting which dye the cells in the marked row.

Maybe the help could be more clear for this feature.
But this enhancement wish should also provide how this feature could be extended without changing the default/actual behaviour. Maybe this could be solved better as an installable extension.