Whatsapp backup and not enough storage space

Hi

I’m trying to back up my WhatsApp chat if 30GB but it keep prompting me that “not enough iPhone storage, free up xxGB to back up”

the backup is to iCloud and my iCloud hv more than 1TB free storage space. Even if back up to my phone, I have more than 100GB available storage space on my phone which is 3x more than the required backup space.

its very confusing and frustrating cos no matter what I do (delete data etc) the message will keep coming out.

I have also tried to restart my phone but still same issue.

anyone experience the same thing or know any solution?

thanks in advance!