@Veshv @esotericgravity I had a look and you both were right, it wasn't in the air poker chapter.
The panel I'm referring to is literally just one small black panel with an unknown voice saying something like "not my child" or "give me back my child" - it was part of a larger dream / flashback and very easy to miss. It may have happened much earlier in the manga, perhaps before Protoporos began.
I only started reading a couple months ago but I remember this clearly because I made a comment on the bottom of the chapter itself. Unfortunately it was on a different site and not easy to find. I will try to find it for you guys when I get home tonight.
Edit: I may have confused it with the air poker chapter because there was another silhouette smoking a cigar in that flashback, which I had thought was (also) Baku's father.
Edit 2: Somehow I was really sure it was part of Baku's nightmare, after Yakou and Bai Long had theirs. But now I noticed Baku's "nightmare" wasn't shown anywhere near that chapter (420 - 423?). So perhaps it's in a more recent chapter? Really hard to find one tiny panel when the narrative goes back and forth in time so much...
Edit 3: I stand corrected. The panel I had in mind actually said "Marco... return him", which I think is just a dream about the grudge of Baku's victims (https://mangadex.org/chapter/204680/2). However, I still have yet to find that silhouette with a white suit and cigar which made me suspect about Baku's father, will update here when I come across it after a re-read (eventually).