...I can't be the ONLY person to make tree farms, with fences... Right....?
Fences typically don't, I've used fences to wall off tree farms for a long time.
It appears to be an issue with both of the new fences actually, I just tested them all. All fences still block palm tree growth, only Wrought Iron and Bamboo fence block other tree growth. I added a screenshot, the two ungrown Mahogany trees are backed with bamboo (left most) and wrought iron (rightmost).
The third ungrown regular tree has lead fence backing, and did eventually grow. I didn't notice it hadn't grown yet until just now, added those after the others so they had less time to grow ><