We have identified that Text Blocks appear as pop-ups since Moodle 4.4 (they were embedded in previous versions of Moodle). You can compare both situations in the attached screenshots.

Since this update, it's not possible to write inside the formula editor, even though we can open MathType accordingly. The focus is always redirected to the Text Block popup. However, after we forced the deletion of the Text Block popup and the Add Block scroll list, the input returned to normal.

It seems that the popup prevents our editor from gaining the user's focus, making it impossible for our editor to obtain the user's keyboard input.