I've been migrating Trello cards, testing first on phab-01. The cards I created from February 19 onwards aren't viewable by others, e.g. https://phab-01.wmflabs.org/T697
The card history has:

Maniphest changed the visibility of this Task from "Public (No Login Required)" to "Custom Policy". Via Old World · Thu, Mar 5, 7:25 AM
Maniphest changed the edit policy of this Task from "All Users" to "Custom Policy".

(Chasemp wasn't sure what Via Old World means.) Back in January Maniphest on phab-01 didn't reach in and change card settings, e.g. https://phab-01.wmflabs.org/T488

I can reproduce by filling in the form at https://phab-01.wmflabs.org/conduit/method/maniphest.createtask/ . I suspect the fix is to explicitly set viewPolicy and editPolicy in the conduit request to some undocumented value, but phabricator.wikimedia.org doesn't need this step. Or maybe my import user "trellimport" needs different settings.

The workaround is to edit each task and change its "Visible To" and "Editable By" fields; sadly I can't do this in Maniphest query's batch editing mode.

(This isn't high severity or priority for my work.)