Consider the following code:

import sys

def trace(frame, event, arg):
    pass

def f():
    f()
sys.settrace(trace)
print(sys.gettrace())
try:
    f()
except RuntimeError:
    pass
print(sys.gettrace())

When I run it, I get

<function trace at 0x108865158>
None

Apparently, the infinite recursion somehow resets the trace function.  This interferes with the coverage tools.

It looks like this has been known for quite some time.  See issue10933.