Not yet anyway, and unfortunately it sounds so intermittent that it may prove really difficult to track down.
However, what is not mentioned is what the phone is connected to. You posted in the PBX section; yet did not mention any PBX.
You indicated the issue would resolve itself for 4-5 weeks after a reboot and that you were running a version of firmware that was released mid-November (121) This begs the question if there are other phones of the same model and firmware experiencing the issue and how long has the system been installed and was this an issue prior to the .121 release.
In the meantime take a look at the release notes for the firmware. Do a search for “hold”. There are some settings that you might want to examine that pertain to how hold is implemented in conjunction with the PBX.