I have core v25 connected to my fulcrum server with sparrow wallet.
Hmm, If it's Fulcrum, then there shouldn't be a "mirrored" watch-only wallet in Bitcoin Core since it doesn't work like EPS or Sparrow's direct connection to Bitcoin Core.What I'm thinking is you might have previously connected Sparrow to Core using either options (not Fulcrum) which created a watching-only wallet.
Thus the wallet where you're seeing the sending transactions.
But now that you're using Fulcrum, your new descriptors from the other (new?) hardware wallet aren't imported to that existing watch-only wallet.
Any tips would be appreciated!
If the above scenario is correct, you can manually import your new wallet's public descriptors to that watching-only wallet.You can get it from Sparrow Wallet: "Settings Tab->Export>Output Descriptors"
Then import the Bitcoin Core-compatible descriptors to Bitcoin Core using importdescriptors command.
Thank you! I managed to fix it with a Core reinstall before I saw your reply, but the logic of *how* to fix it was bugging the heck out of me. Your reply makes a lot of sense so I think it would have fixed it, but unfortunately I won't be able to find out for sure
Nonetheless you've finally put my mind at ease trying to figure out what the problem was haha. 
