There is a lot going on here with context (or lack thereof) and intended meanings.
Also, try different version of this sentence like "This bike is stolen"
Is the bike in front of you? Do you or who you are talking to know where the bike is?
If the bike is right there in front of both of you, then "My bike is lost" or "This bike is lost" make no sense at all. And "My bike is stolen" means something like "I stole this bike".
If the bike is not in front of the two of you, then "My bike is lost" and "My bike is stolen" are explanations of what it is not there. Both sentences are legitimate but would be more naturally said as
I lost my bike.
and
Someone stole my bike.
Trying to extract a consistent meaning from a single sentence is not always easy - real world context should disambiguate.