I don't know why it happens but it happens.
Normally, I would tag all the contents and then insert the form fields. And then I would go back to the Reading Order Pane and fix it. And then, I would turn on the Read-Out-Loud function and listen to it. It would normally read as tagged.
But every now and then, it wouldn't read any of the form fields' tool tips (it normally does). If I specifically select the selected form field (via mouse or tab), it would read. But it wouldn't read the form fields as per the Reading Order Pane.
It's even reading Background'ed objects.
I figured out how it happened, not why though.
Possibly corrupted file?
Basically, there were a PowerPoint slide that "bugged out". Random parts of the slide popped up under Untagged Content in the Full Check but it's neither in the Tag Structure Pane or Content Pane. So I decided to delete the page and insert a new one. And that's what caused the problem.
I also tried just clearing the content in Edit and pasting the slide back in. It resulted in the same problem.