Hey Ian,
I will need a bit more information, answer my questions below and I'll take a look into this.
1. What QB version are you using?
2. Are you creating this client in Method?
3. Is it messing up after you click save?
4. Is it messing up after the sync?
5. Is it only showing like this when you fill out an invoice?
Let me know those answers and I can try to replicate this, thanks!