How do you input quarter notes to form chords without their being tied together?

See https://musescore.org/en/handbook/3/note-input#chords

Or https://musescore.org/en/handbook/3/voices ?

Are you perhaps using real-time MIDI? That's the only situation I can think of in which ties result in unexpected ways. The fix themselves when you complete entry of the measure.

If that's not it, and reading the handbook doesn't show you what you need to know about how to enter notes, please describe in more detail what you mean - the exact sequence of keys you are pressing of buttons you are clicking, and the step at which an unexpected tie appears. Ideally. also attach the score you are working on.