Did the T-Mobile tech confirm to you both phones located in the exact same physical locations are being assigned to the exact same frequency bands? Just having them connect to the same cell tower is not enough.
For example, the iPhone 14 supports the following 5G NR frequency bands:
n1, n2, n3, n5, n7, n8, n12, n14, n20, n25, n26, n28, n29, n30, n38, n40, n41, n48, n53, n66, n70, n71, n77, n78, n79
The Galaxy S23 appears (Samsung themselves don't specify) to support:
n1, n2, n3, n5, n7, n8, n12, n20, n25, n28, n38, n40, n41, n66, n77, n78
So if the iPhone 14 is assigned to a frequency in bands n14, n26, n29, n30, n48, n53, n70, or n71, you may see drastically different performance characteristics simply because the S23 cannot use those frequencies.