I was able to reproduce the problem on your example.
I Uploaded the index.cshtml file modified.
The problem seems that happen when a column filtered is a template (I have done a template column for rendering a html link to another page instead just the text)
The first time the filter icons appear correctly.
When I Ungroup the colum that was grouped, the icon disappear.
It tooked some time to reproduce the problem, becasuse on your samples all my thoughts worked correctly, till this one.
Hope it helps.
If there is a quick workaround, I'll ask if you can provide me, otherwise I'll need to put a command button instead of link to redirect the navigation on the other page.
Andrea Perazzolo
Attachment:
Index_e145cde3.zip