Tarde, mas ainda assim, eu acabei de lidar com o mesmo problema, e isso funcionou para mim:
RequestHeader set X_FORWARDED_PROTO 'https' env=HTTPS
A documentação diz:
When the RequestHeader directive is used with the add, append, or set
argument, a fourth argument may be used to specify conditions under
which the action will be taken. If the environment variable specified
in the env=... argument exists (or if the environment variable does
not exist and env=!... is specified) then the action specified by the
RequestHeader directive will take effect. Otherwise, the directive
will have no effect on the request.
Enquanto a variável de ambiente HTTPS é definida somente quando a solicitação é feita por meio de SSL.