Profiling RTS flag `-po` does not work

The RTS flag documentation says this about -po:

empty:   -p         Time/allocation profile in tree format
empty:              (output file <output prefix>.prof)
empty:   -po<file>  Override profiling output file name prefix (program name by default)

However it current doesn't work as advertised in 8.4 and HEAD. When I use -po<file> no file is generated. Reproducer:

$ cat empty.hs
main = return ()
$ ghc-stage2 empty.hs -fforce-recomp -rtsopts -prof
[1 of 1] Compiling Main             ( empty.hs, empty.o )
Linking empty ...
$ ./empty +RTS -poprof
$ ls | grep prof
$

Just -p works:

$ ./empty +RTS -p
$ ls | grep prof
empty.prof
$

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Profiling
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