Cannot enable labs flags which are supposed to be force-enabled via config.json

[richvdh]

Steps to reproduce

1. Enable a labs setting (for example, new room header)
2. Disable it again
3. "Force-enable" the labs setting by configuring it to true in config.json (cf Enable new room header by default on develop #26630)
4. Reload Element

Outcome

What did you expect?

The forced-on setting should be on.

What happened instead?

The setting is not enabled. Worse, it is greyed out and not even possible to enable:

Operating system

No response

Browser information

No response

URL for webapp

No response

Application version

No response

Homeserver

No response

Will you send logs?

No

[richvdh]

As a matter of interest, I rolled back to before matrix-org/matrix-react-sdk#12125 and reproduced the same behaviour