The ViewGraph is an in-memory directed acyclic graph data structure in which nodes represent view definitions and edges represent "view-on" relationships. This structure assigns unique unsigned 64 bit numbers to each node in the graph, using ViewGraph::_idCounter:

https://github.com/mongodb/mongo/blob/6f7fd7318d61bd145bb75a9a0a5d35387d2a6b9f/src/mongo/db/views/view_graph.h#L182

The intention is that concurrent access to this counter is prevented by the ViewCatalog's mutex. However, the _idCounter is a static data member. There is a ViewCatalog per database, each owning and synchronizing access to a separate ViewGraph instance. Since the _idCounter is static, all ViewGraph instances share the same counter! This means that the various ViewGraphs can all access the counter simultaneously. This leads to the assignment of invalid node ids, which in turn corrupts the in-memory graph. We have seen this manifest as a process-fatal invariant failure, or as an unexpected failed view catalog operation (e.g. a view drop, modify, or create).