An inbox message body is not the same as a post body. Inbox messages are truncated by design.
Here's a snippet of my inbox:

Notice how the "body" is truncated at 100 characters? (Counting the ... and accounting for HTML entities like '.)
The results I get from /2.2/me/inbox?filter=!SmOhH2b13NujlyyXX6 are exactly the same:
"body": "Very good. I'm not an expert on user scripts after all. Good job.",
...
"body": "Aah, thnaks. The original answer didn't have that information (as linked) but it's quite ...",
...
"body": "@BrockAdams Ah, that's what I suspected. Sorry, I'm veering from the original question at...",
...
There is no bug here.