Thank you for the report.

I just made sure, that it is no regression. Your example did not work in 4.2.2, 4.4.1, and 5.1.0 either.

To give a short overview:

.
├── @bar        # classdef
│   └── bar.m
├── @foo        # old style class
│   ├── foo.m
│   ├── meth1.m
│   └── meth2.m
└── runtest.m

with

function x = meth1(o)
  z = bar;
  x = meth(z);
endfunction

function x = meth2(o)
  z = bar;
  x = z.meth;  # Error with or without assignment to x!
endfunction

runtest.m

o = foo;
meth1(o);
meth2(o); # Error

The error message:

>> runtest
error: class not found: foo
error: called from
    meth2 at line 3 column 5
    runtest at line 3 column 1