Since 5.5, the preview tab is disabled by default as it's been moved to the actions toolbar on top right (see NXP-7559).
Since 5.8, actions enablement is propagated through contributions, so merging the action does not enable it anymore (see NXP-12675).
This results in not showing the preview tab anymore on some documents, when this is set from Studio.
A workaround is to enable explicitely the action "TAB_PREVIEW" in a XML extension.