Version Used:
VS2019 16.11.10 Preview 1.0
Steps to Reproduce:
(doesn't repro in a console app (at least i don't know which specific requirements are necessary), but reproduces reliably in my project. i'm willing to explain more, but it's not OS.

1. have an overriden method M with a base calls
2. have a local method LM in that method M
3. put a breakpoint in M after the base call
4. debug until breakpoint is hit
5. modify content of LM till EnC error appears
6. notice: {0} and {1} aren't replaced

Expected Behavior:
{0} is replaced with the method name, {1} is replaced with the internal error
Actual Behavior:
this error text is shown:

Modifying the body of '{0}' will prevent the debug session from continuing due to internal error: {1}

Notes
when discussing the behaviour in the c# discord, @Youssef1313 mentioned that that error string isn't in the roslyn sources...