Why esp32 does not enter the 80211n mode when the AP is fixed to 80211n?

[jason2]

ESP32 as station,we use d2wd.Another device(mt7601) as AP.Mt7601 supports 80211bgn.
If AP is fixed to 80211b, all of the transceiver packages between AP and esp32 are 80211b mode.
If AP is fixed to 80211g, all of the transceiver packages between AP and esp32 are 80211g mode.
If AP is fixed to 80211n,all of the transceiver packages between AP and esp32 are not 80211n mode.
All phenomena are based on a wireless network card(rtl8723) in monitor model that captured.It is confirmed that rtl8723 can capture 80211b,g,n.
The attachment if wireshark log.