Good question! I'd ask questions multi-part questions like "What's happened now?" and see if it comes up naturally. You could ask "Is your account open?" and then a follow-up "How do you know?" or "What's making you think that?" – personally I'd capture that outside of the matrix as there's rich insight that might get lost.

Alongside the matrix I like to print off screens and capture participants quotes and findings. Comprehension questions would be better suited to deeper exploration that take a bit more time.