I read something in the manga:
When this battle will finally be over... take me out to eat some tasty yakiniku.
"when...will be" seems strange to me, isn't it better saying "When this battle is finally over"? Since this is translated from Japanese, I am not sure whether its grammar is correct or not.
OP's is quite right. Clauses identifying a future time using when, after, before, as soon as, until, etc. use present tense. Only the main clause (saying what will happen then) uses future tense.
Look at these examples from Ege Academy...
When I finish writing the reports, I will go out with my friends.
When the party is over, we’ll clean the house.
After her sister prepares dinner, they’ll invite their neighbours.
My father will buy a new car after he saves enough money.
etc., etc.
Yes, this is strange, and I think you're probably correct in diagnosing it as a translation flub. It's the kind of use I often hear from non-native speakers.
In the Japanese language, there are 2 main tenses. One for actions that are done/finished (past) and another for those that aren't done yet (present and future). They simply specify time counters to indicate when an action took/takes/will take place. We can presume that the person who translated the manga just had a lapse of judgment here, re: choice between present and future tense.
