On her death in 1937, she is single with no occupation, and she is living in The Philadelphia Hospital for Mental Diseases. This particular hospital was notorious for its corrupt and unhealthy conditions. (She died after having her elbow drained!) A photographer snuck images out showing the conditions, and in 1938 Elenore Roosevelt encouraged the State of Pennsylvania to take charge of the facility and improve its service.

I think that it is worth considering that Miriam may have been institutionalized during the 1930 census. That would not explain her being absent from the census, but it might explain incorrect or misleading data provided by the staff, that is making it hard to find her. I will try to get her admission date, and if that is after 1930, I will look at other similar facilities in the area.