Cannot see expected files and attachments on Account records in Lightning

You may have noticed that some records don't appear under the 'Notes & Attachments' related list on an  Account record when a User switches to Lightning Experience, however the same records are available when they switch back to Salesforce Classic.

Example scenario

• John Test creates an Account record called 'Acme', he adds an attachment called 'Attach 1' to this record.
• John Test now creates a Contact record called 'Acme Contact', he associates it with the Acme account and adds an attachment called 'Attach 2' to this record.
• John Test finally creates an Opportunity Records called 'Acme Oppty,' he associates it with the Acme account and adds an attachment called 'Attach 3' to this record.

When John Test views the Notes & Attachments related list on the Account Record in Salesforce Classic, he sees all 3 attachments, namely:
 

• Attach 1
• Attach 2
• Attach 3

However, when John switches to Lightning Experience, he sees only the attachment associated to the Account record. In other words, John sees 'Attach 1' and not the attachments associated to the Contact & Opportunity record. John should view the attachments separately to the Account, Contact & Opportunity record.

Similarly, the same behavior can be observed when converting Leads with attachments:

1. Create a Lead.
2. Add an attachment to the Lead
   • The newly added attachment appears under the 'Notes & Attachments' related list in Salesforce Classic.
3. Convert the Lead by creating a new Account, or add to an existing Account
4. Open the Account in Salesforce Classic and you will see the Attachment record which was created in Step 2.
5. Switch over to Lightning Experience and navigate to the Account. The attachment does not appear under the 'Notes & Attachment' related list.

Note: Upload a file under 'Notes & Attachments' for a Lead record in Lightning Experience. When a Lead is converted, the file will not be transferred to the Contact or Account. The attachment in Lightning Experience is an action to upload a 'File.' At the moment, Files don't support the roll-up during Lead conversion. Thus, the 'Files' are not visible in the Contact or Account.

Files added via the 'Upload Files' button in the 'Files' related list will also not roll-up (the same is true for Files added via the 'Notes & Attachments' related lists in orgs with the setting 'Files uploaded to the Attachments related list on records are uploaded as Salesforce Files, not as attachments' is enabled).
 

Review the following articles for more details:

• Changes to the 'Notes and Attachments' related list and the buttons 'New Note' and 'Upload Files' 
• Considerations for usage of note-taking tool 'Notes' and 'Files' functionality