Whilst coolant expansion tanks/reservoirs are great when they work correctly, people get used to looking at the level in the bottle and think all is good with the coolant system because there is fluid in the tank.
When they function as desired and the coolant system is completely sealed, they have low fluid level when cold, high fluid level when hot, and low fluid level again when the engine cools down. If you don't watch to ensure the level is changing as noted above, you can be lulled into a false sense of security.
If your coolant system develops a leak, it pushes fluid out the leak/hole when heated, and draws air back in when cool, instead of drawing fluid in via the reservoir (because the system is not sealed). Then the level in the bottle doesn't change. And each time it is heated, more fluid is expelled, more air replaces it and the engine can overheat and seize without the temperature sender showing it.
The leak may be small, and only expel fluid when the engine is operated when temp and pressure goes up (out and about in the field etc), so it is not seen as a leak on the floor under the tractor when you park it up for the day.