I created a custom entity with some functionality, and one of the things I added was the ability to sit. It does sit, but I cannot get it to change/execute behavior when it's sitting. It appears the minecraft:sittable code doesn't work for custom entities:

 
"minecraft:sittable":

{ "sit_event": "minecraft:taking_a_seat", "stand_event": "minecraft:time_to_go" }

 
 
Steps to reproduce, add the following:

Component group to enable sitting:

 
        "buckbeak:tame": {
          "allow_sitting": true,
          "minecraft:sittable":

{             "sit_event": "buckbeak_sitting",             "stand_event": "buckbeak_standing"           }

,
          "minecraft:behavior.stay_while_sitting":

{             "priority": 0           }

}
 

Then the sittable event:

      "events": {
        "buckbeak_sitting": {
          "add":

{             "component_groups": [               "buckbeak_sitting"             ]           }

        }

 
and then the component group (in this case, I want to set it on fire when sitting so I can easily see the code is working:
 

 
      "component_groups": {
        "buckbeak_sitting": {
          "minecraft:is_ignited": {}
        } 
 

Nothing ever happens when you make the entity sit. You can get the animation to trigger, just not the events/component group for it. Other parts, like the "on_leash" event and subsequent component group do trigger.
 

This is on the Windows 10 (bedrock) version