YoYo Games are maintaining their own fork of the Spine runtime, so this is a bit out of our control. They are currently updating to the latest Spine version, hopefully the API will improve.

I checked their API reference here https://docs.yoyogames.com/source/dadiospice/002_reference/game%20assets/sprites/skeletal%20animations/index.html

It appears there is no way to specify looping. The only way I can think of to stop a loop after one iteration is to call the skeleton_animation_clear function. You'll have to keep track of whether the animation has finished based on how much time has elapsed.

I'd suggest to contact YoYo Games and ask them ro provide the functionality you require.