In Model view for the sql endpoint of a lakehouse, model doesn't persist, Mark Date Table greyed out. Using MS Fabric / PowerBI / Synapse Engineering

Question

This is done in MS Fabric / Synapse Data Engineering using a PowerBI Premium capacity.

Following MS's recommended "layered" architecture, I've created a different dataflow to ingest from each source, and then created dataflows to transform the data (linked dataflow, ingesting from the original ingest dataflow). These dataflows are all gen 2, and the second layer picks a lakehouse as the data destination. In the lakehouse, I navigate to teh sql endopint (top right), and then to the model view tab (bottom left). There, I create a model, and also try and mark a date table created in PowerQuery as a date table.

Two problems:

1. The model does not persist - opening the same window again will show the model intact once or twice, but it soon disappears.

2. The "Mark as Date Table" option is greyed out.