Litecoin-cli validates segwit bitcoin address as valid litecoin address
No, this is working as intended. When litecoin first introduced segwit, they also used 3.... adresses. They were (are) generated the same way ( Using the same P2SH as bitcoin.).

These now start with an M.., but because of backwards compatibility these 3... adresses are still valid. (but no longer used.)

I guess this issue covers what you are facing pretty well - https://github.com/litecoin-project/litecoin/issues/433