Can I leave during the petition for I129

I have a question to ask. If someone knows, please help me out. I am working in a company with CPT. My company filed I129 for H1B. We have a business conference in Singapore on August and I was planned to go. But our lawyer told me that I can't leave U.S. until the petition is approved. Is it true? I never heared of this and it doesn't make sense. Thank you for your help.