AP join happens using control plane, as long as AP HA has Primary, Sec controller's name and IP info and have L3 reachability to those HA WLCs(from AP's IP at the remote site) failover happens without an issue. The locally switched wlan mapped to vlan is per site basis.
For hreap, AP holds the WLAN to VLAN map config for locally switched wlan unlike central switch wlan mapped to dyn int of WLC.
It is my suggestion -fail the hreap APs atleast once to all HA WLC before production so that all WLCs can learn the AP config to avoid AP's being mapped to default vlan.