I see what you are saying. Unfortunately this is more a function of how custom attribute filtering works in general. Most custom attribute filters for search pages in TeamDynamix only support attributes which can have choices, like dropdowns/checkbox lists/radio button lists/multiselects.
There are a few places where we have added textbox and textarea filters into the search filters, like Report Builder, ticket searches and TDNext issue searches, but they have never been in the client portal issue searches.
If you would like, I can make an enhancement for to add them to the issue search in Client as well.