Losing Apple Health data when switching Garmin watches

There might be nothing I can do about this but I wanted to check I’m not missing anything or could/should be doing something else.

I swapped my Vivoactive 5 for a Venu 3. Previously using the Vivoactive as the primary device for Apple Health data. During the swap there was about 4-5 hours when I didn’t have a watch ‘active’.

So once I disconnected the Vivoactive, my iPhone tracked distance/steps for the rest of the day. But once I synced my new watch at 9pm I lost previous data and it picked up where the Vivoactive left off.

I thought this might happen so I wrote down the data so I could enter it to Apple Health manually if it did.

So I just wanted to confirm this is expected or could/should I be managing/setting up my data tracking another way?

Thanks in advance!