Omada Controller 4.4.4 running on Linux server

Checking server.log I see there is a lot of messages like this that repeats from time to time and all at the same timestamp.

2021-09-24 10:24:30 [net-event-handler-pool-10] [WARN]-[SourceFile:484] - roaming ap 1X-XX-XX-AA-BB-CC pat stat is null, not issued
2021-09-24 10:24:30 [net-event-handler-pool-10] [WARN]-[SourceFile:484] - roaming ap 1X-XX-XX-AA-BB-CC pat stat is null, not issued
2021-09-24 10:24:30 [net-event-handler-pool-10] [WARN]-[SourceFile:484] - roaming ap 1X-XX-XX-AA-BB-CC pat stat is null, not issued
2021-09-24 10:24:30 [net-event-handler-pool-10] [WARN]-[SourceFile:484] - roaming ap 1X-XX-XX-AA-BB-CC pat stat is null, not issued
2021-09-24 10:24:30 [net-event-handler-pool-10] [WARN]-[SourceFile:484] - roaming ap 1X-XX-XX-AA-BB-CC pat stat is null, not issued
2021-09-24 10:24:30 [net-event-handler-pool-10] [WARN]-[SourceFile:484] - roaming ap 1X-XX-XX-AA-BB-CC pat stat is null, not issued
2021-09-24 10:24:30 [net-event-handler-pool-10] [WARN]-[SourceFile:484] - roaming ap 1X-XX-XX-AA-BB-CC pat stat is null, not issued
2021-09-24 10:24:30 [net-event-handler-pool-10] [WARN]-[SourceFile:484] - roaming ap 1X-XX-XX-AA-BB-CC pat stat is null, not issued

It's more frequent with the AP with MAC address 1X-XX-XX-AA-BB-CC, but happens with them all. This AP is EAP 225 v3 with the latest firmware version (5.0.5 Build 20210604 Rel. 51118)

What does that message indicates? I don't see it in the event logs on the UI... also, don't see any roaming request there as well...