Use MathObjects string comparison methods

Very bizarre: with seed 1611 the correct answers B and A for the first two parts were marked incorrect by str_cmp($ans1,"ignore_case"), etc.  Switching to MathObjects via Compute($ans1)->cmp fixed the bug so that now correct answers are marked correct.  Is there something wrong with str_cmp?  Is there interference between the strings added to the context and str_cmp?  I hope not.  I am using theme: math4 | ww_version: 2.8 | pg_version: 2.8.