RS422 Receiver Not Reporting Errors When Too Few Stop Bits
A framing error is a logic low in the STOP bit position, rather than the correct logic high. The STOP bit position is 'n' bits after the START bit (logic low). 'n' depends on if you're using 8-bit or 9-bit data and if you're using parity:

 9 for 8-bit data, no parity
10 for 8-bit data, parity
10 for 9-bit data, no parity
11 for 9-bit data, parity


Transmitters are usually configured to send multiple STOP bits because the receiver cannot deal with the incoming byte rate and may overrun. STOP bits after the first are pauses in transmission and not part of the byte framing.

Older systems generated an interrupt to a relatively slow CPU for every received byte. Some receivers didn't have input buffer registers, so the next incoming bit would corrupt the byte just received. Bytes took time to unload.

These days, CPUs are typically very fast compared to baud rates and buffering circuitry is cheap, so your average system can keep up with streams with 1 STOP bit between bytes.



EXAMPLE DATASHEET TEXT

NAT SEMI PC16550 (used in huge numbers in PCs in 1990s)


  The Receiver checks the first Stop bit only, regardless of the number
  of Stop bits selected.


NXP SC2692


  After the stop bit is detected, the receiver will immediately look for
  the next start bit. However, if a non-zero character was received
  without a stop bit (framing error) and RxD remains Low for one half of
  the bit period after the stop bit was sampled, then the receiver
  operates as if a new start bit transition had been detected at that
  point (one-half bit time after the stop bit was sampled).
  
  Framing Error A framing error occurs when a...stop bit are zero. If
  RxD remains low for one half of the bit period after the stop bit was
  sampled, then the receiver operates as if the start bit of the next
  character had been detected.