Editing a post that uses a form template, the composer preview renders any multiline composer field as one run-on line: headings glued to the sentence after them, lists and BBCode as literal text. The saved post cooks correctly — only the preview is wrong.
Cause: form-template-field/composer.gjs mirrors each composer field into a hidden <input type="text"> for native validation:
<input id={{this.validationInputId}} type="text" name={{@id}} value={{this.composerValue}} ... class="form-template-field__composer-hidden-input" />
The composer reply — which is what the preview cooks — is assembled from those inputs. Per the HTML value-sanitisation rules, a type="text" input strips CR/LF on assignment, so every newline in the field value is gone by the time the preview sees it. Saving reads the editor itself, which is why the stored raw is unaffected.
Repro (2026.9.0-latest): any form template with a composer field → enter text containing blank lines (e.g. a ## heading, a paragraph, a list) → open edit on the saved post → the preview shows it flattened into a single line while the post itself renders fine.
Workaround we are running as a theme: flip the hidden inputs to type="hidden" (which preserves newlines on assignment), repair the already-flattened value from the field editor, and dispatch one input event so the reply rebuilds. That fully restores the preview.
Note for a core fix: type="hidden" skips native required constraint validation, so the stand-in control probably wants to be a visually hidden <textarea> rather than a text input.