Pretty much what the title says. When looking at their tooltips, I noticed that Selector Tag [0] has material information, but Selector Tags set to something other than [0] don’t.
Here’s a screenshot showing the tooltips for [0], [2], [4], and [9]:
(side note: I like to edit gregtech.lang, so my tooltips will probably look a little different from stock GregTech. I don’t think it’s relevent to this issue, however.)