Have you overridden anything in the OOTB UserPortal portal or using your own portal other than OOTB UserPortal? If so, please check your harness rule and make sure the pz-cosmos-ui-scripts bundle is present as pictured below in the OOTB version.
I checked the OOTB My WorkList table in the UserPotal, and this seems to be configured with Open by handle action as well. Here, preview works just fine (right-hand side expands with preview content). Therefore my assumption is some cosmos related JS may not be getting called properly in your scenario.