if you fill the rad too fast you trap air in the block. the coolant will not flow down the top rad hose into the block and the air will not bubble up. in some circumstances it will not happen even if / when the t-stat opens.
you may have to drain it down part way to release the air bubble and then refill slowly. the coolant does not flow past the t-stat . it has to flow in from the top slowly enough so the coolant can run down while the air runs up, all in the same pipe. once the coolant blocks the flow of air up you are stuck.
if you think about the position of the coolant crossover pipe on top of the block, the coolant has a long way to go to get to the driver side. which means there is a lot of opportunity for an air lock.
the siphon is so slow there is no chance of an air lock, unless you use a really large hose.
i have a ''fill tube'' device i use for adding ATF and emptying oil drain catch basin. you can use it instead of a funnel. it threads on to either a one gallon coolant, ATF, type jug or a one quart oil bottle. it has a 12 inch tube and a valve built in so you can up end it before you release the flow of liquid. i have not tried this as a coolant filler, but since the tube is so small it could work. just don't squeeze the jug to force the coolant out of the jug and into the rad.
this shows it attached to a measuring jug. i bought it without the jug. not expensive.
but the bottom line is open vent plug and fill slowly.