I'm not sure if this particular subject has been discussed previously, but here goes anyway.
An equinox occurs twice a year, once on, or around the 21st of March, and again on, or around the 23rd of September. An equinox is when the sun at midday is exactly overhead the equator. The round earth model accounts for these events because of the 23.5 degree wobble in the earth's axis of rotation throughout the year, and the flat earth theory accounts for this because of the migration throughout the year, of the sun's circular path over the earth, with the sun's path transiting between the Tropic of Cancer in the northern summer, and the Tropic of Capricorn in the southern summer.
So if an observer is located on the equator during an equinox, they will observe in the morning, that the sun appears at first light, moving from an exact easterly position, then moving in an exact straight line throughout the morning until it is directly overhead at midday. At this point the sun will caste no shadows whatsoever. From midday onwards, the sun is now observed to move in an exact straight line towards the west, until it has eventually disappeared at the end of the day.
You will have noticed the blue highlighting of the words 'exact straight line', which has been deliberate. This is to draw your attention to the fact that the observed path of the sun through the sky during an equinox, is always an 'exact straight line', from the time the sun first appears, through to midday and then till the sun has eventually gone at day's end.
The spherical earth model accounts for the sun's 'exact straight line movement, due to the earth's axis of rotation being 'exactly' perpendicular to the sun on the 21st of March and 23rd of September.
Now we come to the flat earth model, and a big problem arises.
According to the flat earth model, the sun makes a circular path once a day over the earth's surface.
I will say that again - 'The sun makes a CIRCULAR PATH over the earth's surface.'
I will now ask a flat earth proponent to explain how the sun, which is making a CIRCULAR PATH over the earth's surface, can be observed during the equinox, to move in an exact straight line in the sky between morning, midday and evening?
And yes, this observation of a perfectly exact straight-line tracking of the sun is fact, and is observed/celebrated twice a year from the La Mitad del Mundo monument located in Peru, which is situated exactly on the equator. Of course, this observation can be made at any point on the equator during either of the two yearly equinoxes.