I'd call this a bit strange. There might be a good reason, but I'm not sure:
%album% HAS TT
is case sensitive and returns not much more than THRaKaTTaK.
%album% IS *TT*
is insensitive and hits a lot.
Obvious why and how questions arise.
"HAS" is not all the way case sensitive.
foobar2000 v1.4 introduced Unicode asymmetric search for some of the matching operations. Apparently "IS" was left out.
Under "HAS" semantics, typing "TT" matches "TT" but not "tt"; typing "tt" matches both "tt" and "TT".
I'll fix the inconsistency for the next update.
Also there have already been usual complaints about the new behavior so there's a switch to revert to 1.3 semantics buried deep in Advanced Preferences.
I was one of those who nagged you to get the asymmetric search, so pleeeeease keep it.