Cannot receive the data when data length exactly 64 or 128 bytes

I tried to write the data to Arduino MKR Zero but always read 0 (Serial1.println(SerialUSB.available());) when data size exactly same as 64 bytes or 128 bytes. send 256bytes are fine.

If I work around this by issue another transaction to send the last bytes of 64/128byte data.

```
#Host side
if len(msg)%64 == 0 and len(msg) != 256:
self.comm.write(msg[0:len(msg)-1])
self.comm.write(msg[len(msg)-1:])
```

Does anyone meet this issue?
Thanks

Start by reproducing USB serial transfers of exactly 64, 128, and 256 bytes on the Arduino MKR Zero, checking SerialUSB.available() and comparing them with the split-write workaround shown in the issue. Trace the SAMD21 serial receive path to determine why the boundary-sized transactions are not readable; done means 64- and 128-byte messages are received without an extra transaction.

Written by the indexing model from the issue text.