Appears to be a re-surfacing of the old autoleveling companion bug. Treesinger druids plant companion works fine for the first few levels, but when the druid hits level 7, companion stops leveing with her, and can not manually add companion levels to the companion either.
I did notice that manually adding a level of PLANT to the companion works, and once this is done, if the druid levels again to level 8, the companion once again auto gains a new level of Companion (for a total of 7 companion, 1 plant). However, adding the 1 level of plant is not a good workaround as it results in the addition of additional HP, skill ranks, and a feat that the companion should not receive.
Windows 10 (multiple different computers)
Not a bug, never was a bug, but people not understanding how companions work. Companions do not gain HD every level the druid does. Companions start with 2 HD and Cap at 16 HD. Ergo, you have a few level where no HD are gained. Review the companion progression chart and realize Master/Druid Level and Companion HD are totally separate.
Anyways, compare the code to the chart and you'll see it's spot on: