The Developer Tools feature does not natively support any language other than JavaScript.
It "simulates" (in a very loose sense of the word) support by reading the source maps that the JavaScript specifies and shows the originating code (TypeScript, in your case) instead of the resulting code (JavaScript).
Because of this, there are a few factors that could be at fault here. For example, perhaps the source map was not created correctly (off topic here), or the Developer Tools feature reads the source map incorrectly, or the debugger does not correctly map the code.
If you could add to the created issue, as little code as possible to reproduce the issue, the cause could be investigated and perhaps fixed (if it is not a source map creation issue). If the cause is determined to be a faulty source map, you can file an issue with whatever tool that created that source map (TypeScript, perhaps, unless you use other tools, like Uglify, or Closure Compiler and friends, in which case either one of them could be at fault, or it might be a configuration issue, in case of source map "chaining" - where one tool creates a source map that is being fed to another tool which changes the source map after its processing).