Take a look into the note to check if the title is in the very first line of the note. In bear a title is defined as the first line of the note. If that line is blank then the note is considered as untitled.
It seems that Quick Open won’t search for a keyword if the title is longer than a certain length and the keyword is after that length limit. E.g. I tried typing “abc” to search for the note “lorem ipsum lorem ipsum lorem ipsum lorem ipsum lorem ipsum lorem ipsum abc” but Quick Open doesn’t show it, but if the note’s title is “lorem abc” then it will show the note.