Preconditions and environment

The Invoice model module-sales/Model/Order/Invoice.php can return both a collection or an array. If the invoice object has the 'comments' data key, it will just return it, regardless of it being sometimes an array, like when $invoice->getComments() was called before attempting to add a comment. One reason for this would be that both getComments() and getCommentsCollection() are using the same InvoiceInterface::COMMENTS data key to get/set data, but using different data types (array vs collection) to populate it.
Example to replicate:

$invoice = $this->invoiceRepository->get($invoiceId);
$commentsCollection = $invoice->getComments();
$invoice->addComment('whatever', false, false);

So a call to $invoice->addComment() is causing Error: Call to a member function addItem() on array in /var/www/html/vendor/magento/module-sales/Model/Order/Invoice.php:716.
Comparing getCommentsCollection() method between Invoice and CreditMemo models, reveals the different implementation.

For the above example, adding $invoice->unsetData(InvoiceInterface::COMMENTS); right before the addComment call, will make it work, because due to not having the comments key will make getCommentsCollection() return a proper collection.

Affected versions: 2.4.7, 2.4.8 (I've only checked and confirmed the code issue on these versions)

Steps to reproduce

Implement the following sequence in a custom module:

$invoice = $this->invoiceRepository->get($invoiceId);

$commentsCollection = $invoice->getComments();    
// will set the invoice object data key InvoiceInterface::COMMENTS with an array value

$invoice->addComment('whatever', false, false);
// will further call getCommentsCollection() which will return the array from data key InvoiceInterface::COMMENTS
//   on which ->addItem() will be called, causing the error

Expected result

Comment is added to the invoice

Actual result

Error: Call to a member function addItem() on array in /var/www/html/vendor/magento/module-sales/Model/Order/Invoice.php:716

Additional information

No response

Release note

No response

Triage and priority

• Severity: S0 - Affects critical data or functionality and leaves users without workaround.
• Severity: S1 - Affects critical data or functionality and forces users to employ a workaround.
• Severity: S2 - Affects non-critical data or functionality and forces users to employ a workaround.
• Severity: S3 - Affects non-critical data or functionality and does not force users to employ a workaround.
• Severity: S4 - Affects aesthetics, professional look and feel, “quality” or “usability”.