Refill coolant failed to add to normal volume?

Hello, there looks to be two coolant air bleed locations on this vehicle. The one you know about and then one above the coolant (water) pump. I'll post a diagram of the instructions. There are also vacuum kits you can buy for helping to make sure all the air is out of the system. But I think trying to bleed the system by having both bleeders open when the vehicle is cold in the morning will help..

These steps are not in the exact order only because the steps in between are just the different coolant types and closing the drain on the bottom of the radiator.
but I usually do this when the vehicle is cold, that way there's no opening things when the system is under higher pressure and comes shooting out all over the place.
With the system filled, it is still cold and both bleeder screws have been opened and closed in order after the cap is on (and make sure you put the cap on correctly with the arrow facing the overflow tube).

You shouldn't have too much air building up like that. If you still do, I would wait until it cooled down enough that it's not going to shoot hot coolant everywhere, maybe the next day, and run the vehicle with the cap off, and watch for any bubbles or sign that there may potentially be a head gasket issue. That would be the next concern if pressure is building up so high that the coolant hoses start to expand so much.

Here are a couple guides to help:

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

https://www.2carpros.com/articles/head-gasket-blown-test

It depends on the height of each bleeder, it looks like by the service information they need to be done in that order, as for why there was a pressure difference, I am not exactly sure.

A bad coolant pump will not cause a buildup of pressure like what you are describing. If you had a bad coolant pump the vehicle would overheat while it's sitting still idling.
If the coolant has not been changed in a very long time, there could be some build up around or in one of the bleeders. In the coolant system after so long there will be a type of sludge build up from a reaction that's pretty technical. I'm not sure how the old coolant looked when you drained it, but if it has been in there for years sludge will build up. But an over pressuring issue (if the air is properly bled out) points more to a head gasket that might be starting to leak into the coolant system.

There is a chemical test you can buy at an auto parts store that will test the coolant for any combustion gases. It has a funnel type tube that goes into the radiator opening and the testing fluid in it will turn a certain color if there are any gases from the cylinders being pushed into the coolant system. It's the easiest test to do. Without having to spend a lot of money. Here is a link to one, so you can get an idea of what it looks like. Harbor Freight has a cheaper one for $25 and the fluid for $10 is a purple fluid.
Hopefully it's not a head gasket problem. But if the coolant temperature sensor was bad, you may not have known it was over temperature.

https://www.napaonline.com/en/p/BK_7001006?cid=paidsearch_shopping_dcoe_google