On new Heading blocks, pressing H5 or H6 won't change the Heading level.
This seems to be a visual issue, since changing to HTML mode, the h6 is properly represented.

To reproduce:

• Run Gutenberg on iOS example app or WPiOS.
• Create a new Heading block.
• Select the H5 or H6
• Check that the placeholder won't change size.
• Write some text.
• Check that the text size has not changed to represent the new Heading level.
• Select other Heading levels, the text won't change size.