It turns out that there has been a problem with the Hedera Testnet since July 31st, which results in the exact same issue I am having.

Investigating - We’re investigating an issue on the Hedera Testnet affecting smart contracts. The behaviour was first reported starting 31 July 2025.

Deployments may succeed, but you may experience:

• Contract bytecode not appearing on Hashscan
• Read/function calls failing with a BAD_DATA error