[**UPDATE**: This problem is fixed now.  Upgrade to the latest version of the Mosaico extension.]

This issue is tracked [on Github](https://github.com/veda-consulting-company/uk.co.vedaconsulting.mosaico/issues/237).  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](https://github.com/veda-consulting-company/uk.co.vedaconsulting.mosaico/issues/237#issuecomment-760423831) 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.