Triggers created from trigger prototypes prevent unlinking and clearing the template, if the trigger prototype is depending on a master trigger from the template. 

Host H

Template T

    Trigger parent

    Trigger prototype proto depending on parent

T linked to H

During LLD, triggers are created on H:

    Trigger proto_instance (created from proto)

When trying to "unlink and clear" T from H, an error is raised that it cannot be unlinked since proto_instance are depending on proto.

Steps to reproduce:

1. Clone default Zabbix template "Linux by Zabbix agent" to "Linux by Zabbix agent - clone 1"
2. Under dicovery rule "Network interface discovery" , change Trigger "Linux: Interface {#IFNAME}: Link down" , under trigger Dependencies , set it depend on trigger "Linux: Zabbix agent is not available"
3. Add template to a test host (lucky)
4. Wait until data comes in , discovery rule process data and creates trigger prototypes
5. Make sure that Trigger prototype is created and depends on "Linux: Zabbix agent is not available"
6. Edit host , choose template "Linux by Zabbix agent - clone 1" do click "Unlink and clear" , when  click "Update" button  on host  , get error message 
7. Get error message "Cannot update host"
   Cannot unlink template "Linux by Zabbix agent - clone 1" from host "lucky" due to dependency of trigger "Linux: Interface eth0: Link down". [zabbix.php:17 → require_once() → ZBase->run() → ZBase->processRequest() → CController->run() → CControllerHostUpdate->doAction() → CApiWrapper->__call() → CFrontendApiWrapper->callMethod() → CApiWrapper->callMethod() → CFrontendApiWrapper->callClientMethod() → CLocalApiClient->callMethod() → CHost->update() → CHost->validateUpdate() → CHostBase->checkTemplatesLinks() → CHostBase->checkTriggerDependenciesOfHostTriggers() → CApiService::exception() in include/classes/api/services/CHostBase.php:401]
8.  

Result:

Error message is shown,Template is not unlinked.

Attached screenshot

Expected:
Expected template to unlink and clear data

Workaround:

Remove dependency in template trigger prototype
Execute Network discovery rule
Make sure trigger does not depend on "Linux: Zabbix agent is not available" anymore under Data Collection->Host->test_host->Triggers
Unlink and clear

Notes

Tested on versions 7.0.22 and 7.4.5. 

Same problem appears on 7.4.5 , but slightly different message  
"Cannot update host"
Cannot unlink template "Linux by Zabbix agent - clone 2" from host "h1" due to dependency of trigger "Linux: Interface eth0: Link down".

Screenshot attached: