Is the new belt a genuine Honda part?
Are you 100% sure all the guards, guides, etc. are intact?
Does the belt jump off as you release the clutch, engage the PTO, or under what exact conditions?
Honda did issue a Service Bulletin way back in 1998 for "Drive Belt Comes Off" and it applied to all H2013 models.
The fix (might be) installing an
additional belt guide:
The add-on guide is Honda Part Number 06224-771-305 GUIDE KIT, BELT MSRP $22.72.
Not sure if it comes with instructions, but even as an FYI, drop me a private message with your email and I'll send you a copy of the bulletin.