In a Real Application Cluster (RAC) setup, the SCAN listener running on Node 1 was able to register only the local instances, but failed to register remote instances from other nodes. In contrast, the SCAN listener on Node 2 functioned as expected and was able to register instances from both Node 1 and Node 2.

Below is the status of scan listener 1

lsnrctl stat LISTENER_SCAN1

 LSNRCTL for IBM/AIX RISC System/6000: Version 19.0.0.0.0 - Production on 02-APR-2021 07:12:45

 Copyright (c) 1991, 2021, Oracle.  All rights reserved.

 Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=LISTENER_SCAN1)))
 STATUS of the LISTENER
 ------------------------
 Alias                     LISTENER_SCAN1
 Version                   TNSLSNR for IBM/AIX RISC System/6000: Version 19.0.0.0.0 - Production
 Start Date                15-NOV-2021 20:29:58
 Uptime                    0 days 0 hr. 6 min. 35 sec
 Trace Level               off
 Security                  ON: Local OS Authentication
 SNMP                      OFF
 Listener Parameter File   <grid_home>/network/admin/listener.ora
 Listener Log File         /oracle/orabase/diag/tnslsnr/Node1/listener_scan1/alert/log.xml 
 Listening Endpoints Summary...
   (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=LISTENER_SCAN1)))
   (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=10.xx.xx.xx)(PORT=1521)))
 Services Summary...
   Instance "-MGMTDB", status READY, has 1 handler(s) for this service...
 Service "ORCL" has 1 instance(s).
   Instance "ORCL1", status READY, has 40 handler(s) for this service...
 Service "_mgmtdb" has 1 instance(s).
   Instance "-MGMTDB", status READY, has 1 handler(s) for this service...
 Service "gimr_dscrep_10" has 1 instance(s).
   Instance "-MGMTDB", status READY, has 1 handler(s) for this service...
 The command completed successfully

Below is the status of scan listener 1

 lsnrctl stat LISTENER_SCAN2

 LSNRCTL for IBM/AIX RISC System/6000: Version 19.0.0.0.0 - Production on 02-APR-2021 07:14:48

 Copyright (c) 1991, 2021, Oracle.  All rights reserved.

 Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=LISTENER_SCAN2)))
 STATUS of the LISTENER
 ------------------------
 Alias                     LISTENER_SCAN2
 Version                   TNSLSNR for IBM/AIX RISC System/6000: Version 19.0.0.0.0 - Production
 Start Date                15-NOV-2021 20:36:16
 Uptime                    0 days 0 hr. 1 min. 0 sec
 Trace Level               off
 Security                  ON: Local OS Authentication
 SNMP                      OFF
 Listener Parameter File   /GRIDHOME/oracle/app/product/grid/19.3.0/network/admin/listener.ora
 Listener Log File         /GRIDHOME/oracle/orabase/diag/tnslsnr/Node2/listener_scan2/alert/log.xml 
 Listening Endpoints Summary...
   (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=LISTENER_SCAN2)))
   (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=10.xx.xx.xx)(PORT=1521)))
 Services Summary...
 Service "ORCL" has 2 instance(s).
   Instance "ORCL1", status READY, has 40 handler(s) for this service...
   Instance "ORCL2", status READY, has 40 handler(s) for this service...
   Instance "-MGMTDB", status READY, has 1 handler(s) for this service...
 Service "gimr_dscrep_10" has 1 instance(s).
   Instance "-MGMTDB", status READY, has 1 handler(s) for this service...
 The command completed successfully

To further resolve the issue, we configured separate TNS entry using all SCAN listeners and set remote listener parameter referring below support document.
SCAN Registration Issue: Services Fail to Register to SCAN Listeners (Doc ID 1448717.1)

 REMOTE_LISTENERS_SCAN=
     (DESCRIPTION =
       (ADDRESS = (PROTOCOL = TCP)(HOST = 10.xx.xx.x1)(PORT = 1521)) 
       (ADDRESS = (PROTOCOL = TCP)(HOST = 10.xx.xx.x2)(PORT = 1521))
       (ADDRESS = (PROTOCOL = TCP)(HOST = 10.xx.xx.x3)(PORT = 1521))
     )

 alter system set REMOTE_LISTENER='REMOTE_LISTENERS_SCAN' scope=both sid='*'; 
 alter system register;