I recently downloaded 6.0.2 and started a fresh new world in survival.
I discovered that in the grove, no matter what saplings are planted (by player or villager), only oak trees will grow.
The only other mod in use was JourneyMaps.
I hope this information will be useful. Keep up the good work!
*edit- to expand on this information, saplings of other types are planted, but when they grow, regardless of the sapling, an oak tree is produced.
*edit- upon further investigation, it seems to affect Japanese groves, but Norman groves are unaffected. Haven't tried other cultures' groves yet.