The MAX/OPT flight levels displayed on the FMS PROG page should definitely be achievable at the gross weight entered into the FMS. However, I do recall seeing another user reporting a similar behaviour previously, and that was caused by a third-party weather program generating unrealistically high temperatures at high altitudes. So the first thing that comes to mind is to double-check that the temperature is within approx. IAS +/-15℃ at high altitudes.
Aside from that, we'd really need more information before we could provide a more definitive response. If you see that behaviour again, could I kindly ask you to raise a support ticket with Just Flight Support at the following link: https://www.justflight.com/support
In the ticket, could you please include any screenshots/videos of the behaviour and attach the log file generated by the product. That will ensure our support team has enough information to be able to assist further.
Instructions for locating the log file can be found here: https://support.justflight.com/en/support/solutions/articles/17000152594-where-to-find-the-log-file-s-
Mark - Just Flight