Describe the bug
I have a dynaconf settings.yaml file with layered environments however, I am unable to overwrite/merge some of the parameters using environment variables.

To Reproduce
Steps to reproduce the behavior:

1. Having the following config files:

Your .env content here

default:

	PARAM_1: ''

	docker:
		some_config:
			config_a:
				some_param: 'cde'
				another_param: 'abc'
			config_b:
				different_param: 123
		PARAM_2: ''
		PARAM_3: ''


production:
	PARAM_1: 'param_1_for_prod'

	docker:
		PARAM_2: 'param2_for_prod'
		PARAM_3: 'param3_for_dev'

development:

	PARAM_1: 'param_1_for_dev'

	docker:
		PARAM_2: 'param2_for_dev'
		PARAM_3: 'param3_for_dev'	

2. Having the following app code:

from dynaconf import settings

settings = Dynaconf(
    envvar_prefix='MY_APP',
    merge_enabled=True,
    environments=True,
    default_env='default',
    env='production',
    env_switcher='MY_APP_ENV',
    ignore_unknown_envvars=True,
    settings_files=['settings.yaml', '.secrets.yaml']
)

3. Executing under the following environment

export MY_APP_DOCKER__PARAM_2=my_param

Expected behavior
I would expect that the setting settings.docker.PARAM_2 would be equal to my_param but for some reason this does not work.

The strange thing is that

export MY_APP_PARAM_1=another_param

does work.

The following also works, but for PARAM_3 it does not.

export MY_APP_DOCKER='@merge {PARAM_2=my_param}'

Environment (please complete the following information):

• OS: OSX Big Sur (11.3)
• Dynaconf Version: 3.1.5
• Frameworks in use: -

Additional context
Add any other context about the problem here.