Ovulation symbol disappeared, or changed day
Natural Cycles bases its prediction for your future ovulation on your average ovulation day, cycle length and all previous data. When your temperature has risen enough to confirm ovulation, the actual ovulation day will be confirmed according to the temperature curve. The app may remove the symbol when it believes that you have ovulated, and are awaiting the temperature increase. It may also change ovulation day if your newer data indicates that it might have occurred on an earlier/later day.
If you are planning a pregnancy, you get an indication of fertile days up until ovulation is confirmed, to not miss any fertile days. If you are past the predicted ovulation day, you will not see the ovulation symbol in the predictions.
"I did an LH test that showed positive, but the Natural Cycles showed that my ovulation is much later than this?"
A positive LH is not enough to confirm that ovulation has happened, nor that it will happen. A positive LH test is a strong indicator that ovulation is close, but a positive LH test needs to be followed by a temperature increase. Since progesterone is released when the egg is released and "warming" the body, the temperature increase is thus a very important indicator that ovulation actually occurred and on what day it occurred.
This is a very important function for people with PCOS, since one can get so-called PCOS "false positive" LH rises where LH levels rise in the body but the egg is not released (which also means that no temperature increase occurs because the progesterone is not released).
Read more here to find out how Natural Cycles can know whether you've ovulated or not.