This is expected behavior since Bitcoin Core doesn't purge orphan/stale block, even when it only store the header. On Bitcoin testnet, Bitcoin Core even show far more chaintips.
This shouldn't happen since various LTC testnet explorer show 26XXXXX as latest block height[1-2]. I'm not familiar with LTC, but release note[3] on newer Litecoin Core shows there are few consensus and P2P change. You might want to update your node to latest version and see whether the problem persists.
P.S. don't forget to backup to your wallet file and optionally whole LTC testnet blockchain.
[1] https://chain.so/testnet/ltc
[2] https://blockexplorer.one/litecoin/testnet
[3] https://github.com/litecoin-project/litecoin/releases