The TEfficiency constructors for fixed or variable bin size are not appended to the current directory. The description of TEfficiency mentions that this should only happen for the constructor using two histograms (passed and total).

The following code does not save the TEfficiency object to the file in version 6.24

TFile* file = new TFile("test.root", "RECREATE");
TEfficiency* eff = new TEfficiency("eff", "eff", 1, 0, 1);
eff->Fill(true, 0);
file->Write();

Executing in version 6.18 (latest I have installed other than 6.24) does produce the expected behaviour. After adding eff->Write(); the TEfficiency will also be written in v6.24.

I think the problem is the use of
TDirectory::TContext ctx(nullptr);
in TEfficiency.cxx (e. g. line 733), which was, as far as I see, implemented in version 6.24. I think in the scope after TContext the current directory is nullptr which is also passed to Build(name, title); as well.

Setup:

ROOT v6.24 built from source
OS: Ubuntu 20.04