I have been running into the same issue with QuickBooks Online modernized invoice templates, and after a lot of troubleshooting, I have finally found a workaround that works! It’s frustrating that we no longer have the option to easily add blank lines or adjust the layout as we did before, but this little trick has helped me deal with it:

By using a special invisible character (like a space or a non-printable character) in the blank lines section, it forces the template to keep the spacing as intended. Although QuickBooks has not fully addressed this yet, this simple workaround has saved me a lot of time!

QuickBooks definitely needs to make this feature more customizable again. But for now, if you are dealing with a similar issue, this might be the solution you need.