AI Pro subscribers throttled in API

So I've had a Gemini AI Pro subscription for a while now. I do some light development work and the Antigravity that comes with the plan is absolutely fantastic and sufficient for my needs.

I recently wrote a script that uses AI for inference for various different analyses. I was using it with the DeepSeek API and it seemed to work just fine. But since I was so impressed with Gemini 3.8 Flash, I decided to get an API key and use my quota from the AI Pro plan instead.

To my surprise, most, but NOT all, of my AI inference requests to the API are blocked with a response saying the server is too busy right now and I should try again later.

I separately purchased API credits from AI Studio and used that API key instead. That worked flawlessly.

I know the AI Pro plan is not meant for API-based inference, but it is supposed to include some basic quota / Free Tier. I am absolutely not able to use that with my plan.

Anyone else have a similar experience?