This is a consequence of the paste_remove_spans option being set to true. Bold and italic text are marked up using <strong> and <em>, but underlining is done using inline styles in a <span>, and these are stripped when pasting.

Can't see any easy solution, as changing paste_remove_spans to false will mess with pasting from word processors.

Im closing. if anyone thinks we should only enable paste_remove_spans on paste from word please reopen.

Changing to wontfix — this isn't invalid, we just don't intend to address it. That said, I wonder if it is possible to keep underlines but discard all other spans. Going to ask azaozz to take a look.

Think wontfix is proper here. It's much better to clean up pasted content, no matter where it comes from.

The only way this could work is if we enable the <u> tag. It is once again ​valid in HTML 5 after being deprecated in HTML 4 and XHTML 1, however its purpose/description has changed:

The u element represents a span of text with an unarticulated, though explicitly rendered, non-textual annotation...