[UPDATE: This problem is fixed now. Upgrade to the latest version of the Mosaico extension.]
This issue is tracked on Github. In short:
This is a bug in Mosaico itself, which has been fixed - but CiviCRM isn't using the latest version of Mosaico. Crowdfunding for an update is happening.
If you know what you're doing, you can manually patch Mosaico. I've done so for versions 2.5 and 2.6, but not 2.7. I tried making a patch you could apply but unfortunately it doesn't seem to patch cleanly. I might make a zip file of the patched Mosaico 2.6 for folks to download.