I know… obvious question, is the app updated and are you updated to the latest iOS?

Yes :)

It was happening to me but it’s now fine again

The time when it froze for me was when I had imported WhatsApp chat backup to my new phone. I tried everything - app updates, app reinstall, move my number and chats from Whatsapp to Whatsapp Business and vice versa. Nothing worked until I decided to let go of the chat backup and make a fresh start. Maybe there was some issue with the backup itself.

I won’t recommend this as your data would get lost forever, but if there is no other option and you are fine with the data loss then give it a shot. It might work.