It's false!
Then this means that this address does not belong to your wallet.
Maybe this address is from that seed, however in a different derivation path. It is a legacy address, so we don't have to worry about segwit derivation paths. Maybe in a different account?
@AndreRoos, I would create a new wallet and try those derivation paths and insert that ismine("1YourAddressHere")
m/44'/0'/1'/0
m/44'/0'/2'/0
m/44'/0'/3'/0
....
I would try a few...
I think you didn't answer also what version of Electrum are you using on Tails. Is it 4.0? There is no 4.0, it is a phising.
Edit: I would also try one more thing.
Create a new wallet, insert your seed, click on Options and
check "BIP39 Seed". Insert the ismine("1YourAddressHere").
Now do this:
Create a new wallet, insert your seed, click on Options and
UNcheck "BIP39 Seed". Insert the ismine("1YourAddressHere").
Does the passphrase has to be the same as from the wallet where the address is missing?
How many of m/44'/0'/1'/0 should I try? - The last digit has to be 0 or it can variate as well?
Electrum version: 3.3.8 - which was on the website of Electrum the newest!?