Not a Bitcoin Core user and never has this experience.
To my knowledge if someone have compromised your wallet, didn't they can just spent all the coins you have now? so I think your computer is the problem. Why you not try to use other device and input the seed phrase, I think as long as it never connected with your computer, the device should be safe.
maybe its an older wallet and those are changes?
Other people would experience the same thing like you if Bitcoin Core was the problem, AFAIK there were no complaints.