Some clients cannot associate when the machine acts as an access point
An access-point operator found that some client devices could not associate. After tracing, the cause turned out to be a single wrong comparison in the wireless stack's transmit path.
Frames injected by the access-point daemon were not being tied to the correct virtual interface before sequence numbers were assigned. Correcting the comparison fixed association for the affected clients.
Takeaway. Association failures that affect only some clients can come from a one-line logic error. Packet captures on both sides help narrow it down.