In the current implementation of altar search:

An altar is searched from the sacrificed mob's feet position up to two blocks downward and one block upward. This makes it impossible to sacrifice mobs from directly below the altar; since most mobs are two blocks high, the altar right above them would always be out of range.

On an unrelated note the FTB Wiki for the Dagger of Sacrifice also incorrectly states that the range is a centered $5 ^ 3$ cube, which led me to make my current design in-game just for me to realize that it wouldn't work. :(

Should we expand the search upward by one block to make bottom-feed designs viable?

Edit: this affect versions from at least 1.12 to 1.16.

FTB Wiki is not an official source for documentation. Any change to this behavior would be in a supported version (1.18.2 at the time of writing) only.

@WayofTime Should I create a pull request to alter the behavior? Personally I think a "base altar" should not be fed from below (gravity and all) and would prefer something that provides the functionality instead (you wanted to exand Arrays, right?).