But the issue is: his current seed phrase (he's positive that it's the correct one) can't restore his previous wallet.
He mentioned that he had upgraded his "E_Tails" using the appimage; and he believe that it's the corruption bug based from the issue's replies (in GitHub).
With those info, it could be the corruption bug.
I read the GitHub comment about file corruption caused by deleting the wallet file from outside the app. I don't see how it fits this story. TAILS is amnesiac, all files are deleted on shutdown, so that bug could only affect a wallet created within a TAILS session. The workflow - create wallet, write seed phrase, copy & paste address to Windows Electrum, send transaction - would have to be fairly contorted to write the empty wallet's seed phrase and copy&paste the other wallet's address. Maybe I'm overthinking, and the out-of-app file deletion is exactly the contortion which would cause this
There is also his claim that he successfully restored the wallet with seed words in TAILS before installing the Appimage. I'm inclined to believe this. If it's true, then the file corruption problem is irrelevant, whether it occurred or not
I suggest the seed phrase he's using now is from a fresh wallet created with the 3.3.8 Appimage, an empty wallet. The seed words for accessing the coin in the older version Electrum wallet are somewhere else, or lost. This contradicts his claim that he has the right seed words. In the absence of proof and 19 months passage of time, it's more likely than any other guess, in my opinion