I've noticed on some of my Ogg vorbis files that Amarok does not display the (full) title, as easytag shows it.

Looking at the files with ogginfo or vorbiscomment shows that there are several TITLE lines in the comment. Easytag interprets the following comment example as "Fernando Sor - Estudio Op. 6 no 2", while Amarok shows only "Fernando Sor".
After reading the definition for the vorbiscomment format, I know that it is allowed (and even encouraged) to have several definitions of, for example, ARTIST (all artists are listed on their own line).

An example comment follows:

TITLE=Fernando Sor
TITLE=Estudio Op. 6 no 2
ARTIST=Narciso Yepes
ALBUM=La Guitarra Clasica Espanola
DISCNUMBER=1
DATE=1989
TRACKNUMBER=07
TRACKTOTAL=18
GENRE=Classical

Why does not Amarok show the full tag? Is this how it should work? Should easytag change their code?

Ogg does indeed support and encourage one tag per logical entity.  If a song has more than one artist, it should have more than one ARTIST tag.

Amarok should list the song once for each ARTIST associated with it.  It doesn't, but it should.

As for TITLE, probably there should be a preference for how to display multiple TITLEs, like what separator string to use when joining them together for display, e.g., " - ".