Shuttle sent your stock update to Etsy, but left this listing's SKU as it was on Etsy and on Shopify.
Why it happens
On Etsy, this listing keeps one stock number for each value of a single option. For example, there is one quantity for each Size, and every Colour of that Size takes from it. While stock is shared like this, Etsy cannot also keep a different SKU for each variation.
Shuttle will not change your stock layout to make room for the SKUs, because that would show more stock on Etsy than you have. So it sends the stock update as normal and skips only the SKU change. You can check this on the listing in Etsy: the quantities are set for one option, not for every variation.
The SKU is not copied back to Shopify either, so your Shopify SKUs stay as they are.
How to fix it
You only need to do something if you want each variation to have its own SKU on Etsy.
In Etsy, open the listing and edit its quantities.
Change the quantity settings so every option affects quantity. Then each variation has its own stock number.
Enter the stock for each variation. Your shared stock is split into a separate number for each variation, so check every number.
Save the listing.
After that, Etsy can keep a separate SKU for each variation.
Still seeing it?
If you see this notice in the Stock Syncer page after every variation on Etsy has its own quantity, contact support. Tell us the listing's name.