When using TMaps within a function in blueprints & then compiling the blueprint causes a warning to be created in the output log. In some test cases it only occurs with a local function TMap variable; however, in the test project provided, the warning can be shown with a blueprint TMap variable.
The error that is being shown is:
LogScriptSerialization:Warning: Error: Unknown bytecode 0x3C; ignoring it
I have also provided a screenshot of the warning
This issue does not occur in versions before 4.15 because TMaps were introduced in 4.15
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-43899 in the post.