It's resolved. The issue was the port 29814 was not exposed in the Omada Controller. After opening it up, adoption started to work with new versions of the firmware.

@shberge and you were right... thanks! I saw that in the firmware notes but I thought that since my Omada Controller and the AP are in the same subnet without a firewall, there could be nothing...

I've submitted a bug report. Maybe someone will notice it: https://community.tp-link.com/us/home/forum/topic/560226?page=1

According to reports from various users in the thread https://community.tp-link.com/en/business/forum/topic/529086, all types of EAP225 (V1, V3, Outdoor V1, etc.) have reports of failed adoptions...

Exact same issue here - cannot adopt AP with Omada controller after upgrading the AP firmware: EAP225-Outdoor V1 (EU) - upgraded to 5.0.7 (but tried with 5.0.8 as well) Omada Controller 5.1.7...