[DerekM]

Sounds like you're editing an OPML file. At the document picker, tap the Edit button, select that file, tap the Share button (the top left one), and select Convert to OmniOutliner. A copy of the document will be made in the OmniOutliner format which will save your formatting. OmniOutliner can edit and save OPML files but that file format does not support any styling.