This seems like an old 'issue', as so many users have complained about it over the years. Actually, it isn't an issue at all, since you just need to go to
Configure -> Preferences -> Document Classes and set 'Word wrap long lines" for all the classes you wish (not just the Default class!). Particularly, since TP automatically opens a new text document on startup, you need to set the option for the Text class.
Try this and let me know
Word-wrapping as default behaviour when TextPad is launched
Moderators: AmigoJack, bbadmin, helios, Bob Hansen, MudGuard
Word-wrapping as default behaviour when TextPad is launched
I'd like to change the world, but they won't give me the source code!