Thank you for making a bug report!
Seeing as there is not replay code It is difficult to make a judgement as to whether or not this is actually a bug of the game working as intended.
Something to note about this is that Archfiend Gilfer is a “when” card effect. This means that it is possible for it to miss its timing to successfully activate.
A “When” card effect must be the last thing to happen timing wise. This means that card effects that say something to the effect of “discard, and then if you do” can cause it to miss timing, even if it is done as chain link 1. Looking at the image shown. If you used Veil of Darkness with Archfiend Gilfer the timing would work as follows:
Veil of Darkness activates during your draw phase, Resolves as Chain link 1, Reveal Gilfor, send it to the graveyard, then draw a card.
The last thing that happens is you drawing a card, not Gilfor getting sent to the graveyard. This means that the chance for Gilford has passed and it missed its chance to activate as its condition was met during the resolution of a chain and you cannot activate cards once a chain begins resolving.
If you had something like Danger! Mothman! and discarded Danger! Mothman!, because the card text says “If this card is discarded”, it doesn’t miss timing, and you can activate it after the chain has resolved.
In addition to this, if you activated Danger! Mothman!'s effect when discarded, you’d be able to draw a card, and then discard a card. Because of the wording, if you chose to discard Archfiend Gilfer at THIS point, because it is the last thing happening, it will allow you to activate Gilfer as it is the last thing to occur, and is at a point where it can be activated on a new chain.
If you disagree with this, and can supply us a with a replay code so that we can confirm if this is working incorrectly, then we will be happy to look into this again. Additionally, we can discuss this on the Duelist Unite Discord should you wish to have a further discussion.
If you agree with this, please mark this as the solution and the thread will close shortly after the final comment is made.
Once again, thank you for making a bug report! If you find anything else you believe to be a bug, please feel free to open up another report!