In version 2.9.19 there was a bug - hotspot walled-garden rules did not work.
That bug was introduced in 2.9.19 and fixed in 2.9.20.
Thanks to hecklertm for reporting a problem!
Why is the walled garden fix not posted in the 2.9.20 changelog?
The changelog for 2.9.20 is :
*) fixed write-sect-total for RB500;
*) added limit of simultaneous hotspot mac logins to 1 for each MAC address;
*) made LCD displaying more stable;
*) added WinBox support for routing-test package;
*) fixed PPTP client to not disconnect every 30s on some occasions;
*) fixed bug - interface and queue graphs got lost on reboot (introduced in 2.9.19)