Fortunately the action of disabling a function causes an Azure Activity Log to be written; ("entity": "/subscriptions/subscriptionid/resourceGroups/gyourRG/providers/Microsoft.Web/sites/yourfunctionapp/functions/yourfunction/properties/state")
Because of this, you can create an alert in the event that someone disables your function.
https://learn.microsoft.com/en-us/azure/azure-monitor/alerts/alerts-logic-apps?tabs=send-email
I'd encourage you to check your function app's Activity Log and find out who's been disabling your functions 🤪
The other thing i'd consider is locking down who has access to your resource group, or even deny assignments