MS Teams .net Bot SDK does not contain a property for 'Subject' and this seems to be disregarded by the bot even with RSC permissions to read all messages.
IMessageActivity
and IActivity
I thought it may be added as an attachment with the html supporting it but this is also not the case.
(Secondary issue is the attachment is not created unless the text contained has a level of formatting)
There is no way to get the 'Subject' line from a chat message in Microsoft Teams using the .NET Bot SDK.
If you wish you can suggest this feature here: Microsoft Teams Community