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)