A bit of a variation on this issue.
Our finding is that a dashboard filter on a merged query dimension from one query doesn’t seem to work (no suggestions found). In other words, the dimension is only found in one of the two models involved in the merged query.
Also, even if the user types in the value in the filter, it doesn’t really filter the tile. In the column for the filtered dimension, it removes all the other responses, but does leaves the row, with a null value for the filtered dimension.