It seems the Pop-Up ignores HTML formatting.
Compare the TOU shown in AGOL's item page...
To the custom license pop-up showing the same info in Open Data...
Is this a bug or a feature?
Unfortunately the custom TOU pop-up only displays plain text. We're about to start feature work for enabling content managers to edit and select suggested and custom TOU from within Hub so I will add this as an enhancement request to our backlog.