Hi Gunter Ohrner, thanks for reporting this (and the other) issues. You can create a box in two ways:

• using the Style drop down
• using the box macros (either with the / slash command, e.g. /info, or using the + button from the toolbar)

The first option was introduced before we had the ability to edit box macros inline. Back then it was a pain to edit box macros, you had to go through the macro wizard. For this reason we introduced these styles, to make it easier, but it's not the same thing:

• The "Info Box" entry from the Style drop down is a block style, so it applies to blocks. Paragraphs are blocks, so you can apply "Info Box" style to a paragraph. When you press Enter inside a paragraph it creates a new paragraph which copies / preserves the styles, including the "Info Box". This is the expected behavior.
• Box macros behave differently because they define a nested editing context, like a nested document (the box macro content), where you can have multiple paragraphs.