I suspect you simply missed the change addresses when exporting the keys from your core wallet.
So when you sent the BTC out, it would have take all the BTCs from the change addesses as well. However, you've only exported the keys for your 8 "receive" addresses... and not your "change" addresses (FYI, these don't show up in Bitcoin Core when you goto "File -> Receiving addresses" list)
Have a look at the actual transaction(s) where you sent all the BTC from Bitcoin Core to your new Core Wallet on a blockexplorer... how many addresses show there? Did you import the keys for ALL of those addresses into ElectronCash?
That would explain it. Thanks for the tip, I'll check it out tonight and write here, if this solve the problem
Electron cash has 100 inputs limit per wallet as i know.
to solve this, you have to Sweep funds from wallet to your Electron cash wallet, this could take 2 or 3 sweeps if you have many inputs on your wallet.
You can try to follow this step b step tutorial using Electron cash :
https://bitcointalk.org/index.php?topic=2078292.0