You are a newbie and I won't blame your carelessness of using a centralized system to safe your coins. I guess maybe,you must have imported your seed phrase of your blockchain wallet to your wallet. Laziness is what has caused your lost because spectrum wallet is very easy to understand and use.
Online wallets are not safe due to their vulnerability to attacks,you have learnt a hard lesson and I feel for your lost. You use learn how to use electrum wallet to keep your investment safe, try and learn more on the forum,to broaden your knowledge. Not your keys,not your bitcoin.
In all probability it is something I have done, the only thing is I cannot work out what I have done wrong.
In 2021 I could not get access to my bitcoin in Blockchain so I opened a spectrum account/wallet and tried to import my bitcoin to spectrum which never worked at the time and I tried to use my seed phrase to do this.
I am not sure what I have done wrong here and the only place that my seed phrase was exposed to was my spectrum wallet which at the time I thought I must have been doing wrong, it's only now two years later something seems to have happened, maybe my bitcoin is in spectrum?, all I know is that something happened in spectrum on Friday six times where nothing at all has happened in two years and that is the same night my bitcoin left blockchain.
I am probably wrong here as usual but my seed phrase being in my spectrum wallet I cannot see how that is a problem, from what I have heard it is safer, that is the only time ever I tried to use it.