Access Point using NetworkManager only allows 6 devices to connect

I've a Raspberry Pi running Debian 12 (bookworm) and have set up an access point with the following command:

sudo nmcli con add type wifi ifname wlan0 mode ap con-name MY_AP ssid MySSID \
   connection.autoconnect yes \
   ipv4.method shared \
   ipv4.addresses 192.168.5.1/24 \
   802-11-wireless.band bg \
   802-11-wireless.channel 7 \
   ipv6.method disabled \
   wifi-sec.key-mgmt wpa-psk \
   wifi-sec.psk 'PASSWORD

This works fine, but it looks like it's only allowing 6 devices to connect. If I try and connect a 7th, it says it won't connect and nothing appears in the NetworkManager logs to indicate that there was an attempted connection or why the connection wasn't permitted. I've tried connecting different devices in different orders, starting and restarting NetworkManager and playing with config files, but it always stops playing ball after 6 devices have connected. If I look in the NetworkManager logs, I see things like the following:

May 15 12:13:22 farmpi dnsmasq-dhcp: DHCPACK(wlan0) 192.168.5.104 98:f4:ab:e0:b2:26 strip2-4646
May 15 12:13:22 farmpi dnsmasq-dhcp: DHCPREQUEST(wlan0) 192.168.5.104 98:f4:ab:e0:b2:26
May 15 12:13:22 farmpi dnsmasq-dhcp: DHCPOFFER(wlan0) 192.168.5.104 98:f4:ab:e0:b2:26
May 15 12:13:22 farmpi dnsmasq-dhcp: DHCPDISCOVER(wlan0) 98:f4:ab:e0:b2:26
May 15 12:13:16 farmpi dnsmasq: cleared cache
May 15 12:13:16 farmpi dnsmasq: using nameserver 192.168.55.1#53
May 15 12:13:16 farmpi dnsmasq: reading /etc/resolv.conf
May 15 12:13:16 farmpi dnsmasq-dhcp: DHCP, IP range 192.168.5.10 -- 192.168.5.254, lease time 1h
May 15 12:13:16 farmpi dnsmasq: chown of PID file /run/nm-dnsmasq-wlan0.pid failed: Operation not permitted

This seems ok to me (chown error aside, but I assume that's not relevant?). The DHCP IP range should allow more then 244 devices to connect, and the connections are being handled correctly, it's just for some unfathomable reason, it stops doing that after the 6th device. I increased the log level to TRACE to see if I could get any more information, but nothing is printed about devices trying to connect. Any advice on how to try and debug this would be greatly appreciated.