In Mac80211, currentIFS is set to DIFS upon successful reception and EIFS upon reception of a corrupted frame. This is correct. However, currentIFS remains unchanged while performing post-backoff. As a result, the EIFS related to reception of a corrupted frame (and subsequent post-backoff) is used again in the minimum duration of idle time before a transmission is performed (i.e. the mandatory IFS during which a station decides whether to transmit directly or go do backoff). This results in incorrect behaviour, as this is a new opportunity.
A fix is simple: in handleEndContentionTimer(), add "currentIFS = DIFS;" inside the state==IDLE if-clause. This is where post-backoff completes and not only remainingBackoff but also currentIFS should be reset.