It might be related to the fact that cloud items have not been completely downloaded. You could work around this problem by first downloading them in the Files app and them importing in Stage Traxx. But importing smaller batches like you did is also fine.
Today I looked if it would be possible to access lyrics from the Apple Music database and it does indeed work. So version 3.4 will be able to import lyrics from Apple Music even if they are not stored in the audio files but only as metadata in the database 🙂
And even better: lyrics are then written into the audio file so you can copy the file to your computer and the lyrics will be embedded.