I know this question is pretty old but I had the same problem and fixed it.
What you CAN do but SHOULDN'T do is:
$pdf->SetFont('verdana_bold', 'B', 12);
$pdf->SetFont('verdana', '', 12);
What you basically do here is define 2 fonts. One with the name verdana and one with name verdana_bold.
While you specify the B for bold it can't find this ttf. Because TCPDF basically checks for a file in the fonts dir called verdana_boldb.ttf.
This doesn't exist so it takes the verdana_bold.ttf (which at first sight seems to be the correct behavior).
For me the issue got noticable after I tried to use both bold and non-bold styles in a table and I either only got the whole table in bold or the whole table in non-bold (removing or adding the B style specifier doesn't make a difference).
What you SHOULD do:
Add the new font type:
$fontname = TCPDF_FONTS::addTTFfont($fontfile, 'TrueType', '', 32);
When you want to use the font:
$pdf->SetFont('verdana', '', 10, '', false);
When you want items in bold in a HTML cell use the html b tag:
<b>myvalue</b>
You can check in the fonts directory if you have the verdanab.ttf file.
$ ls fonts/verdanab.
verdanab.ctg.z verdanab.php verdanab.z
I hope this helps someone else :)