It looks like a bug on JasperServer 5.5, but I write a question in case of I missed something.
As you can see into the picture below, when a column is filtered, its textual content may be splitted even if the area is wide enough to handle that content on one single line.
It looks like a bug into the PDF export process. When you look at the html representation of that column header, you notice that when filtered, a column header area is splitted into 5 <td> tags (IMHO to cope with alignment consideration). The <td> tag that contains the text fits perfectly the width of its content, as you can see below.
I guess it's a computation failure during PDF export process. Could you please give a hand ? Is there any workaround ?