Hello,
In fact, there is no bug but the confirmation workflow you're using is not the right one.
You can find 6yo threads with the same content and my answer did not change for the past years ; but I do hope that it will evolve in the future.
The thing is : You're confirming the order in the backend order listing.
By doing that, you have a popup and you can see the email content before the order is really confirmed.
When you see the email content, because the order is not confirmed, there is no serial.
So you can't have the HikaSerial content in the email preview.
And that preview is not just a preview, it's the email content that will be sent to the customer.
So !
If you just open the order details in the backend and modify the order status via the normal interface ; you will have a little checkbox to "notify the customer".
That email will be sent after the order been modified (and confirmed), exactly the same way an order is confirmed by a payment plugin.
There is no bug in HikaSerial and nothing it can do.
Since an order is not confirmed, it cannot generate/assign serials to the order (just because the order is not confirmed yet and you might not confirm it after all...)
Since the email content is generate before the real modification, the email content is based on a non confirmed order details.
You see an issue with HikaSerial content but it's not the only possible problem.
Regards,