hostapd: Pi 3 built-in WiFi going up and down every second

I am using a Pi 3 Model B running Raspbian buster and kernel version 4.19.118-v7+. I have been trying to configure it to run as a WiFi repeater using the built-in WiFi (wlan0) which is configured to function as an AP using hostapd. I am also using a WiFi dongle (wlan1) which connects the Pi to the LAN via WiFi.

I found out that the built-in WiFi interface, which is used by hostapd, goes from the "UP" to "DOWN" state every second. I found this out by running: watch -n 1 ip a show wlan0. Below is the output of ip a show wlan0 for both the UP and DOWN states:

wlan0 is DOWN:

    3: wlan0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
        link/ether b8:27:eb:xx:xx:xx brd ff:ff:ff:ff:ff:ff

wlan0 is UP:

    3: wlan0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
        link/ether b8:27:eb:xx:xx:xx brd ff:ff:ff:ff:ff:ff
        inet6 fe80::xxxx:xxxx:xxxx:xxxx/64 scope link tentative                                                                    valid_lft forever preferred_lft forever

Is there a way to troubleshoot this to find out why this is happening?

Below is the output of /etc/hostapd/hostapd.conf:

interface=wlan0
driver=nl80211
ssid=my_network
hw_mode=g
channel=11
wmm_enabled=0
macaddr_acl=0
auth_algs=1
ignore_broadcast_ssid=0
wpa=2
wpa_passphrase=my_network123
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP
rsn_pairwise=CCMP