Font of the run is not specified directly in run only. This is complex value that is inherited from the run’s parent, document styles, themes and document defaults. When you remove run from the document it’s parent is set to null and the code that gets font name cannot get the inherited values and returns value that is directly specified in the run or if it is not directly specified returns default value.
Let’s consider the following three paragraphs. The first is simple bold text with Normal style that uses Calibri font. The second is paragraph with the same normal style, but with directly specified font for run and paragraph. The third is paragraph with a custom ArialStyle style, that has Arial style specified.
<w:p w14:paraId="2787B8CC" w14:textId="607BD13B" w:rsidR="00C96F00" w:rsidRPr="00CA1A63" w:rsidRDefault="00CA1A63">
<w:pPr>
<w:rPr>
<w:b />
<w:bCs />
</w:rPr>
</w:pPr>
<w:r w:rsidRPr="00CA1A63">
<w:rPr>
<w:b />
<w:bCs />
</w:rPr>
<w:t>Bold Calibri</w:t>
</w:r>
</w:p>
<w:p w14:paraId="7F64B104" w14:textId="7CCF8DB9" w:rsidR="00CA1A63" w:rsidRPr="00C75FFA" w:rsidRDefault="00CA1A63">
<w:pPr>
<w:rPr>
<w:rFonts w:ascii="Times New Roman" w:hAnsi="Times New Roman" w:cs="Times New Roman" />
<w:i />
<w:iCs />
</w:rPr>
</w:pPr>
<w:r w:rsidRPr="00C75FFA">
<w:rPr>
<w:rFonts w:ascii="Times New Roman" w:hAnsi="Times New Roman" w:cs="Times New Roman" />
<w:i />
<w:iCs />
</w:rPr>
<w:t>Italic Times New Roman</w:t>
</w:r>
</w:p>
<w:p w14:paraId="0D91CAFA" w14:textId="21A94755" w:rsidR="00CA1A63" w:rsidRDefault="00CA1A63" w:rsidP="007A778A">
<w:pPr>
<w:pStyle w:val="ArialStyle" />
</w:pPr>
<w:r>
<w:t xml:space="preserve">Normal arial style</w:t>
</w:r>
</w:p>
If you remove the run from the first paragraph its font will not be changed because document default is Calibri.
If you remove the run from the second paragraph the font will not be changed because font name is directly specified.
If you remove the run from the third paragraph the font will be changed to Calibri – document default. Because when you remove a run it’s parent is null and code does not have access to the parent paragraph’s style.