Underline formatting is lost when typing after applying underline

Phenomenon

When applying underline formatting to selected text and then continuing to type, the underline formatting is not maintained for the newly typed characters in Firefox on Linux.

Reproduction example

1. Create a contenteditable div.
2. Type some text.
3. Select the text and apply underline formatting (Ctrl+U or through execCommand).
4. Place the caret at the end of the underlined text.
5. Continue typing.

Observed behavior

• In Firefox on Linux, newly typed characters after the underlined text are not underlined.
• The formatting state appears to be lost when the caret moves.

Expected behavior

• Newly typed characters should inherit the formatting of the preceding text when the caret is within or immediately after formatted text.