Same for me. Also beat act 4, do you need to just beat act 3, maybe? I was also using other minor mods, coloredmap and relicstats, but I doubt that they were impacting this.

Edit: There is a known issues list

https://psteamcommunity.yuanyoumao.com/workshop/filedetails/discussion/1610056683/2576571528377248603/

It mentions this bug and a resolution.

"-Unlocking characters when completing an Act 4 kill does not work. You need only to end a run, so abandoning a run at the first floor will suffice for unlocking, but completing Act 4 will not cause an unlock."

I can confirm that this unlocked the next character for me.

I take posts can't be stickied in workshop discussion forums, otherwise I'm sure that one would be stickied.

Either way, this helped me and should help you as well.

yeah man i beat act 3 afterwards and unlocked it. thanks either way :)