corAR1() structure in lme()

I have no idea -- you may have to dig through the nlme source code to
find out (https://github.com/cran/nlme/tree/master/R). nlme is largely
in maintenance? mode at this point -- it's been 6 years since the last
commit, so even if this is undesired behavior (=bug), there may not be
much to be done about it unless you contribute the fix yourself. :(