You may need to try to clean Alerts in your OEM. This method can be helpful when there are alerts that the agent won’t clear clearstate or reset dynamic properties won’t work

Here are the steps

First, log in as SYSMAN or your repository owner.

Run below query

select t.target_name
, t.target_type
, collection_timestamp
, message
, 'exec em_severity.delete_current_severity(''' ||
t.target_guid || ''',''' ||
metric_guid || ''',''' ||
key_value || ''')' em_severity
from sysman.mgmt_targets t
inner join
sysman.mgmt_current_severity s
t.target_guid = s.target_guid;

This will give you information about all the alerts in OEM. Copy the result from the em_severity column that you want to be removed. Run it exactly as it appears.

For example:
SQL> exec em_severity.delete_current_severity('B5281E99AEE1','6E65075DB4B018289','/u01')