I am writing a piece in the key of G# minor (B major). Alto saxophone and baritone saxophone are tuned to Eb, and therefore should display the key signature of D major/B minor, but instead are displaying the key signature Ab major/F minor. The soprano saxophone is a Bb instrument, and should be displaying C# major/A# minor, but instead is displaying Db major/Bb minor. This means that when I copy lines from other instruments onto soprano, alto, or baritone sax, the notes have an unreasonable and unnecessary amount of accidentals. Only the tenor saxophone transposes properly.
You are incorrect about your expectation of transposition for the Eb instruments is wrong. Concert B major is G#/Ab major on an Eb instrument such as the alto saxophone. Since G# requires a double sharp, MuseScore does not support it so it uses the enharmonic Ab as the key signature. The reason for the accidentals is that there are flats in the Alto Sax key and sharps in the Tenor sax and most other instruments' keys. Fors example, MuseScore knows you copied an D# (concert F#) and since there is no D# in the key signature the accidental is placed on any notes copied to the Eb sax.
When you apply a key signature that has an existing enharmonic key signature, like C# major and Db major, you can open staff/part properties and set the preference for sharps or flats. The tenor sax is correct (according to your expectations) because it defaults to C# major as the key signature.
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I am writing a piece in the key of G# minor (B major). Alto saxophone and baritone saxophone are tuned to Eb, and therefore should display the key signature of D major/B minor, but instead are displaying the key signature Ab major/F minor. The soprano saxophone is a Bb instrument, and should be displaying C# major/A# minor, but instead is displaying Db major/Bb minor. This means that when I copy lines from other instruments onto soprano, alto, or baritone sax, the notes have an unreasonable and unnecessary amount of accidentals. Only the tenor saxophone transposes properly.
You are incorrect about your expectation of transposition for the Eb instruments is wrong. Concert B major is G#/Ab major on an Eb instrument such as the alto saxophone. Since G# requires a double sharp, MuseScore does not support it so it uses the enharmonic Ab as the key signature. The reason for the accidentals is that there are flats in the Alto Sax key and sharps in the Tenor sax and most other instruments' keys. Fors example, MuseScore knows you copied an D# (concert F#) and since there is no D# in the key signature the accidental is placed on any notes copied to the Eb sax.
When you apply a key signature that has an existing enharmonic key signature, like C# major and Db major, you can open staff/part properties and set the preference for sharps or flats. The tenor sax is correct (according to your expectations) because it defaults to C# major as the key signature.