Trying to execute summon with (partial) subcommands (those that follow syntax but do not have all the required parameters) often works in ways you don't expect it to.

Try to run:

/execute at @e summon tnt

Nothing happens.

Now try to run:

/execute at @e summon tnt at

or

/execute at @e summon tnt as

or any other subcommand.

Even though the command isn't completed (run ...) the command still succeeds and summons the entity. At the very least you'd expect the sub command to end in

execute ... if ...

for example. This would be valid since you expect if/unless to work as a test. Also execute store should work. However, for summon, it would not make sense to end the command with a partial sub command.

Expected to work

/execute at @e summon tnt

(currently doesn't work. Should spawn once)

/execute at @e summon tnt summon tnt

(currently only spawns once, but should spawn twice.)