they start off very dark and end very light. two different packs, one is brand new, one is slightly old they both do this. i don’t change pressure at all. they’re expo fine tip and this rly infuriates me when writing. the beginning of the stroke is like normal how it should be then it just disappears
That is just how ink works, especially when it moves across a medium like felt on such markers.
The felt tip becomes saturated when not in use so that is why it begins dark.
The rate in which the ink flows from the body to the tip is lower than the amount of ink a saturated tip leaves behind, so that is why it goes from darker to lighter.
You could try to write very slowly and see that the color becomes more uniform, but I don't see the point of doing so in normal situations. It would take too much time to finish writing a sentence.
This ^ If you want consistency, scribble on some scrap to desaturate the nib before you start writing.
Because it's ink on glass
You may want to try different markers like acrylic or liquid chalk markers.