When you create a cadenza for a part, it affects all parts. You will need to adjust the other parts display so they don't show a bunch of unexpected rests. Most likely, you will want to create a whole rest near the middle of the measure and make all other rests invisible in that measure for the other parts.
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https://musescore.org/en/node/117161 ?
I do not know if it will work for only one Part.
When you create a cadenza for a part, it affects all parts. You will need to adjust the other parts display so they don't show a bunch of unexpected rests. Most likely, you will want to create a whole rest near the middle of the measure and make all other rests invisible in that measure for the other parts.