Is not fully synced yet look at the blocks under "getblockchaininfo" and compared it to the current block height from https://mempool.space/testnet

The current block is 4075204 but yours is currently 4075162

I don't see any other reasons, but this current block height in your node seems to be the issue the node should be fully synced.

The weird part is "verificationprogress": 1 it indicates a fully synced.

If the normal transaction works just fine would you mind to add more info if what lightning software or wallet you use to open a channel? If the issue only comes up when opening a channel, then maybe the issue is only in your lightning software or wallet.