But the fact that there is a ballooning UTXO set on testnet does not surprise me at all, as people rarely spend their coins after they are finish testing something. At best, they just return it to the faucet they got it from.
I finally got the data of UTXO growth on testnet after enabling additional index and using command
gettxoutsetinfo "none" block_height| Block height | Time | txouts value (total UTXO) |
| 2415000 | 2023-01-06 17:41:19 | 28309184 |
| 2570000 | 2024-01-02 08:18:03 | 30140226 |
| 2588100 | 2024-04-19 17:32:26 (just before halving on mainnet network) | 49671269 |
| 2810957 | Today (when i write this post) | 121751208 |
Since from beginning of this year, about 91M UTXO has been created. And since just before halving on mainnet network, about 72M UTXO has been created. So it disprove your speculation that it's caused by people who abandoned their tBTC after done with their testing.