On the equinox, the sun rises almost exactly due east as seen from the equator.
On the equinox, the sun rises almost exactly due east everywhere.
On a single line of longitude on the equinox, the sun rises at the same time. This causes a problem for the selected flat map section. There are not many suns. There would need to be many suns. One for each position along the line.
Consider the line of Longitude East 20°. This line of longitude has close to the most land covered from L’Agulhas, South Africa to Svalbard, Norway. The observations along this line of Longitude are easy to confirm either personally or by communicating with people that live near. On the day of the equinox, the sun rises at the same time all along this line of Longitude.
On the equator, at 20°E the sun rises due east.
In Sweden and Norway, at 20°E the sun rises nearly due east on the equinox. Note that suncalc.net and similar utilities report the angle at the first moment of the sun’s upper limb being visible, not the geometric center of the sun which is closer to due east.
In South Africa, at 20°E the sun rises nearly due east on the equinox.
When we place the observation of the sunrise angle for Norway/Sweden on a flat map we immediately see this doesn’t match observations.
The same happens where the observation from South Africa is placed on the map. The map must be adjusted so all observers face the appropriate direction to see the sun rising at the same time at the correct angle.
Similarily, the sunset angles for these locations must be adjusted to match observations.
Here only possible maps top match sunrise and sunset for this single observation for flat earth.
Here is a view of the globe on the equinox. The earth’s tilt is either toward or away from the camera depending on the time of year. Note that this predicts the sunrise will be due east for all points on a line of Longitude.