Hello,
We are already in contact with Adyen in order to perform a plugin upgrade to support SHA256.
Unfortunately, the website which allow to perform signature test is not working (the one we can see in your first screenshot).
It gives false results compared to production and the Adyen support team themselves told us that the test website is not up to date.
At the end, Adyen team is not able to give us any answer to our question regarding the signature content.
So I'm afraid that the only team which could say "where is the issue" is the Adyen support team and I really hope that they will take some time to send us more complete answer than "the signature is invalid".
Without proper answer regarding the signature and without a test website which allow to test the signature ; It won't be possible to provide any update for the plugin which would be stuck with the old SHA1 mode.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.