A mixed vegetable dish that turns out unevenly is almost always a sequencing problem rather than a heat or seasoning one. Different vegetables need different amounts of time and cannot be given it simultaneously.
Density decides how long heat takes
Heat travels through a vegetable by conduction, so dense, low-moisture pieces such as carrot or potato take substantially longer to soften than open, watery ones.
Cutting size changes this as much as the vegetable itself does, since a thick piece of a quick-cooking vegetable can outlast a thin piece of a slow one.
Cooks therefore either stage the additions or cut the slow vegetables smaller to bring the timings into alignment.
Water release changes the pan
Vegetables with high water content release it as they heat, and once enough liquid is present the pan drops to boiling temperature and stops browning.
Adding those vegetables early therefore prevents everything else from developing colour, and the dish turns out stewed rather than fried.
Where browning is wanted, watery vegetables go in last or are cooked separately and combined at the end.
Acid stops softening in its tracks
Tomato, tamarind, lemon and vinegar all stiffen the pectin between plant cells. Vegetables that meet acid before they have softened tend to stay firm indefinitely.
This is why acidic components are held back until the vegetables have reached the texture the cook wants, rather than added with the other liquids.
Salt runs the opposite way, drawing moisture out and helping soften, which is why it often goes in early in a dry preparation.
Colour is a separate constraint
Green vegetables lose their colour with prolonged heat as the pigment breaks down, and acidity accelerates the change considerably.
Cooking them briefly and uncovered limits both the time and the concentration of acids released as steam condenses back into a covered pan.
A dish needing both long-cooked and green elements is generally best handled by cooking them apart and combining them at the last moment.
Why dry and wet preparations sequence differently
A dry sabzi is built to keep pieces distinct, so vegetables go in staged and the pan stays hot enough to evaporate what they release.
A gravy dish has the opposite goal, and vegetables are often added to an already finished sauce purely to cook through and absorb it.
The same vegetables, cut the same way, therefore enter at completely different points depending on which of the two the cook is making.