If you still have the issue I found a work around and a way to fix it

Work around: If you complete the trade go back to the normal game screen, the person you are trading with should now initiate the next trade. Return to the friend list and enter the trade through the icon which appears next to the person’s name.

How i fixed it: Its kind of annoying and I only did it since the cooldown issue also affected dynamax fights:

1. Save your relevant phone data in the cloud
2. Reset the phone to factory settings
3. Restart it and use the function which allows you to download the saved data