Checking the uncatchable tick box in the pixelmon spawner doesn't really make the spawned pokemon uncatchable. This was also the case in 9.1.3

hey there, you sure you on 9.1.4? since your server was 9.1.3 when i checked.

Not sure what you mean with tickbox, but the uncatchable tag works fine.
https://i.imgur.com/OaJXfov.png

After investigating this with the Team, i can confirm its not bugged. You probably got used to the 1.12 spawners where you can hit checkboxes to make them unbreedable or catchable or tradeable. Thats not a thing in 1.16.5 anymore.

You now provide the full spec directly in the pokemon field.

For example:

random legendary:true gr:pygmy uncatchable shiny lvl:70