I have some issues with symbols, too.
But first, let us prove that I am doing it right.
1) Create text element and place an image below it (centered). Group them together and create a symbol of that group. (Symbol 1)
2) Drag and Drop that symbol into the artboard (Symbol 2), click on the "sync" button – syncing should be off.
3) Edit the text content and color of Symbol 2. (not syncing)
4) Enable syncing.
5) Re-center (select both and click in the alignment panel on horizontal center) the text element to the image below it. (Symbol 2)
The image and text should be centered (Symbol 2) and the image of Symbol 1 stays centered to its text element, since I don't change its "centered" position.
What I get:
The image of Symbol 1 changes its position too. (Bug or feature?) If feature, I suggest to implement this. It would be really helpful to keep symbols centered (only through the alignment panel).
But I am not done:
6) Click on sync button, again (sync off).
7) Center image and text of Symbol 2.
Position should not be synced and the image of Symbol 1 stays at position.
What I get:
Image of Symbol 1 changes evenly with Symbol 2 position, even with disabled syncing.
I hope the issue is clear. OS X El Capitan 10.11.6, iMac (end 2013), Affinity Designer 1.5.1