Lost activity history after changing Apple Watch and iPhone

I owned an Apple iPhone 6 and an Apple Watch 2.

All my Apple watch activity, including Exercises, were stored and displayed in the activity app on the iPhone. All my Apple devices are backed up on a 200GB iCloud storage.

A few month ago, I broke my Apple watch. So I changed it for a new one (same series 2) and pair successfully the new watch with my iPhone 6.

Activity history recorded from my "old" and broken Apple watch was still displayed in the Activity app on the iPhone.

So everything was fine until...

I bought an iPhone 8. So I backup my iPhone 6, unpaired my watch, restored the most recent backup on the new iPhone and relink my watch.

I thought all was going well until I realize two bad things in Activity App:

• The Exercise history has lost the maps of my running training (but not the other data like distance, speed... just maps)
• Even worse, all my activity history from my PREVIOUS Apple watch (the broken one) is gone!

How can I get my activity back?

Unfortunately, I reinstalled my old iPhone 6, so I no longer have the original data (just the iCloud backup but it is like it was not a real full backup!?)