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AP advertised its basic rate set as following, don't support 6M:

Tag: Supported Rates 12(B), 18, 24, 36, 48, 54, [Mbit/sec]
    Tag Number: Supported Rates (1)
    Tag length: 6
    Supported Rates: 12(B) (0x98)
    Supported Rates: 18 (0x24)
    Supported Rates: 24 (0x30)
    Supported Rates: 36 (0x48)
    Supported Rates: 48 (0x60)
    Supported Rates: 54 (0x6c)

And Macbook pro has associated the AP successfully, which means it knows and accepts the supported rates.

But the macbook pro constant sent RTS with 6M rate:

802.11 radio information
    PHY type: 802.11a (OFDM) (5)
    Turbo type: Non-turbo (0)
    Data rate: 6.0 Mb/s
    Channel: 52
    Frequency: 5260MHz
    Signal strength (dBm): -54dBm
    Noise level (dBm): -95dBm
    Signal/noise ratio (dB): 41dB
    TSF timestamp: 162236858
    [Duration: 52µs]
IEEE 802.11 Request-to-send, Flags: ........C
    Type/Subtype: Request-to-send (0x001b)
    Frame Control Field: 0xb400
        .... ..00 = Version: 0
        .... 01.. = Type: Control frame (1)
        1011 .... = Subtype: 11
        Flags: 0x00
            .... ..00 = DS status: Not leaving DS or network is operating in AD-HOC mode (To DS: 0 From DS: 0) (0x0)
            .... .0.. = More Fragments: This is the last fragment
            .... 0... = Retry: Frame is not being retransmitted
            ...0 .... = PWR MGT: STA will stay up
            ..0. .... = More Data: No data buffered
            .0.. .... = Protected flag: Data is not protected
            0... .... = Order flag: Not strictly ordered
    .000 0001 0001 1010 = Duration: 282 microseconds
    Receiver address: Cisco_ba:33:ee (2c:33:11:ba:33:ee)
    Transmitter address: Apple_4a:24:e0 (f0:18:98:4a:24:e0)
    Frame check sequence: 0x0c2d050b [unverified]
    [FCS Status: Unverified]

This behavior caused the macbook pro constant ping loss. Why it sent RTS with a rate that doesn't belong to AP's basic rate set?

BTW: the macbook pro's version is 10.14.2, and 10.15.1 has the same problem as well. The AP model is Cisco AP2802i.