What is the issue?
PDF/UA > Logical Structure > Structure Tree > Formula structure elements > Possibly inappropriate use of a Formula structure element
What does it mean?
Some formula tags in your document are not used formally correctly. In most cases it is due to the placement attribut. Innately the formula tag is an inline level element. If it is used as a block level element, the attribute "Placement: Block" is required. Otherwise some assistive technologies are not able to present it correctly.
How to fix it?
Use the the tool Fix Placement in axesPDF.