The value of a Parameterised lookup changes back to the first value in the list of values in Modern UX. Validating the database and Classic UI the value is displayed properly.
Release : 15.9.x, 16.x
Resolution is to re-structure the Parameterised lookup query to use the @BROWSE-ONLY: tag. Once that is used values are displayed properly in Modern UI.
Another possibility is that the hidden key of the query is not unique. In such cases, we observe an issue where the Grid view of Modern UX shows a different value while the details page shows a different value. In such a case, please contact support for analysis and resolution.