`-fdefer-diagnostics` doesn't work in GHC 9.4.2

Summary

-fdefer-diagnostics no longer works in GHC 9.4.2

I created a reproduction repository here: https://github.com/parsonsmatt/defer-diagnostics-reproduction

Steps to reproduce

git clone https://github.com/parsonsmatt/defer-diagnostics-reproduction
cd defer-diagnostics-reproduction
# set GHC 9.4.2 
make warnings-deferred
# observe that warnings are interleaved in output

Expected behavior

I would expect it to defer warnings and errors, like it does in GHC 9.2 and before

Environment

• GHC version used: 9.4.2, 9.2.4

Optional:

• Operating System: Ubuntu
• System Architecture: x86