Salesforce’s Note object is being superseded by its ContentNote object. A limitation of the newer notes is that they don’t automatically roll up under the associated account when added to contacts and opportunities.

Here is some code I’ve written to solve this (also includes case notes):

Apex Trigger

trigger ContentDocumentLinkTrigger on ContentDocumentLink (after insert) {
    Set<ID> ids = Trigger.newMap.keySet();
    List <ContentDocumentLink> docLinks = [SELECT LinkedEntityId,ContentDocumentId,ContentDocument.FileType 
                                           FROM ContentDocumentLink WHERE Id IN: ids];
    for (ContentDocumentLink docLink : docLinks) {
        if (docLink.ContentDocument.FileType == 'SNOTE'){
            String linkedId = docLink.LinkedEntityId;
            if(!linkedId.startsWith('001') && !linkedId.startsWith('005')){
                String sNoteId = docLink.ContentDocumentId;
                NoteLinkHelper.linkToAcct(sNoteId,linkedId);
            }
        }
    }
}

Apex Class

public class NoteLinkHelper {
    
    //helper method to get account ID related to non-account record from which note was created
    public static String getLinkedEntityAccount(String noteId, String linkedId) {
        String acctId;
        if (linkedId.startsWith('003')){
            List<Contact> contacts = [SELECT AccountId FROM Contact WHERE Id =: linkedId];
            if (!contacts.isEmpty()){
                if (contacts[0].AccountId != null) {
                    acctId = contacts[0].AccountId;
                }
            }
        }
        else if (linkedId.startsWith('006')){
            List<Opportunity> opps = [SELECT AccountId FROM Opportunity WHERE Id =: linkedId];
            if (!opps.isEmpty()){
                if (opps[0].AccountId != null) {
                    acctId = opps[0].AccountId;
                }
            }
        }
        else if (linkedId.startsWith('500')){
            List<Case> cases = [SELECT AccountId FROM Case WHERE Id =: linkedId];
            if (!cases.isEmpty()){
                if (cases[0].AccountId != null) {
                    acctId = cases[0].AccountId;
                }
            }
        }
        return acctId;
    }
    
    //helper method to link note to an account
    public static void linkToAcct(String sNoteId,String linkedId) {
        String acctToLink = getLinkedEntityAccount(sNoteId,linkedId);
        if (acctToLink != null) {
            ContentDocumentLink sNoteLink = new ContentDocumentLink();
            sNoteLink.ContentDocumentId = sNoteId;
            sNoteLink.LinkedEntityId = acctToLink;
            sNoteLink.ShareType = 'V';
            insert(sNoteLink);
        }
    }
}

I hope this helps your users get a more comprehensive view of their accounts.