FWIW, here’s the official word from the Tandem site:
Tandem pumps and cartridges are only indicated for use with Novolog and Humalog U-100 insulins. Importantly, clinical studies using our automated insulin delivery algorithms were only conducted using Humalog and Novolog U-100 insulins, which have different times of onset compared to Fiasp.
If you use Fiasp in your pump and you begin experiencing persistent issues that can be attributed to insulin compatibility, we can’t guarantee your warranty will be honored.
Do you have plans to expand your pumps’ Indications for Use to include Fiasp?
We do not have immediate plans to seek approval for use of Fiasp in our pumps.
https://support.tandemdiabetes.com/hc/en-us/articles/360050655014-What-insulin-can-I-use-in-the-t-slim-X2-pump-