how can a service like ChipMixer use your keys? you don't give them your keys. they are stored on your computer inside your wallet!
and what do you mean by "session" that you deleted?
You've been advertising ChipMixer for a while now, but it seems you have no idea how it works. I think you should at least test it to know what you advertise. In short: ChipMixer gives you private keys, and ChipMixer has a "session" you can delete when you're done.
In my mind, it's looks like Electrum bug, the private keys was imported to the watch only wallet, and after the shutdown wasn't saved. It worked always, hundreds of times.
A watch only wallet, by definition, only holds addresses, and no private keys.
My session deleted with the chipmixer, otherwise i can get back my private keys and import again.
I take it you didn't make a backup of your wallet? I'd never trust just one hard drive when it comes to private keys.
I can't wrap my mind around what happened exactly. A balance can show up on a watch only wallet, but it shouldn't disappear without any transactions. Things just don't add up.