Lake05 You need to enable Send notifications to the current user option.
Settings - System - User it is at the bottom.
You need to enable it for all the profiles you want to receive notifications including the owner profile.
Example
Owner profile(let's call this profile1) - Whatsapp installed and send notification option enabled.
First user/secondary profile(same but it's profile2) - Signal installed and send notification option disabled.
Second user/secondary profile(profile3) - You are here.
Now, when someone text you on Whatsapp, you receive a GrapheneOS notification that says Notification from Whatsapp for Profile1 along with a button that says Switch to Profile1 to quickly go to that profile.The notification won't have any other text like the contact name or the message. You need to swith to profile1 to see who texted you.
For Signal, only way to know whether you recevied any text or not is to switch to the profile(here it's profile2), because the send notification option is disabled on that profile.