Looks like i found the answer myself.
It turns out that there is workspace limitations in opera. And my workspaces were not merged because the first PC has already 5 workspaces ... Oh what a ridiculous limit ?! Looking forward to 73 version release where this limit will be increased.
When i turned off workspaces in PC-2 i got all my tabs back(but now unsorted in previously created workspaces).
Once again, very weird and confusing limit ...

I have 2 PCs.
Some workspaces in 1 of them and another workspaces on the second PC
Decided to sync them.
Create account, turn sync on first PC.
Turn sync on another PC, enter previousy created login\pass.
Here I expected that my workspaces will be merged. But workspaces from second PC are ovveridden with workspaces from first PC ! Is that expected behaviour ?!
On the https://www.sync.opera.com/tabs in section "open table" my workspace tabs are present from second PC but they are actually absent. I don't see neither tabs nor worspaces on second PC