Multiple Relations Trouble
The relatedTo parameter can take an object, an element ID or an array of element objects and/or IDs. In your case the latter is expected so wrapping your arguments in an array should fix the issue:

{% set entries = craft
                .entries
                .section('survivorAssets')
                .relatedTo(['and',{ targetElement: theme },{ targetElement: survivor }])
                .orderBy('assetOrder')
                .all() %}