i was able to fix this. what i did was removing the waba from odoo (by going to whatsapp application in odoo, configuration, business accounts).

afterwards, i created a new whatsapp business account, with the same data. this time it did create a new webhook code, (unlike when just changing some of the parameters like account number or business number, which wouldn't create a new code)

apparently, webhooks can get 'stale' and need to be re-created sometimes, this was a tip i got from customer service.

needless to say, this is a workaround or last resource, when all the rest of requirements have already been met, such as missing some of the many verifications that meta asks for, payment menthod, or alike, which would result in the same behaviour

cheers