The "cheap" fix would be to change line 76 of xdebug_tracing.c from:
xdebug_str_addl(&str, " => ", 6, 0);
to:
xdebug_str_addl(&str, " => ", 8, 0);

That would fix the output, though it doesn't fix the fact the function_stack_entry->level is off by one. The level could just be considered one deeper for the purposes of this function, changing lines 73-75 of the same file from:
for (j = 0; j < i->level; j++) {
xdebug_str_addl(&str, " ", 2, 0);
}
to:
for (j = 0; j < (i->level+1); j++) {
xdebug_str_addl(&str, " ", 2, 0);
}

Where xdebug_return_trace_assignment() is called in xdebug_code_coverage.c, the deepest stack frame is used for the first parameter. I'm not sure why this frame is not sufficiently deep. It may have something to do with the point at which variable definitions are determined from the Zend engine.

Hope this additional info helps!

Fixed for 2.2.1 and 2.3 on github.