Attempt to reproduce the problem:
Use V3 to insert diagram analogous to what I see in V2:
Entered this FEN: 8/8/4k3/8/6R1/3K4/8/8 w - - 0 1
Selected blue board, classic pieces
Added a comment to the Setup: { in this position with White to move there are 14 rook moves White can play which retain the winning position. There are also 8 moves of the White king which retain the winning position. So a total of 22 different moves White can play and win. }
OK, so the above worked OK in V3 and displays in V2. Now, I have switched to V2, and am going to insert the same diagram:
Select "Diagram", click Continue
Paste the same FEN, click Continue
Insert diagram caption, same as mentioned above.
OK, so the problem is that diagrams entered in V2 do not display properly in V3. This bug was reported several months ago. I will test a workaround next, by entering the same diagram in V2 and then editing the post in V3. In the past, simply opening the diagram and saving it again used to work OK.
OK, so the workaround is still applicable. Alas, it works only for my own posts, and will not let me fix the posts made by others when using moderator's toolset.
Here's an example of a thread that was written by GM @severomorskij in V2, and is useless in V3: Weaknesses of chess engines -- and the V3 source HTML does not even seem to contain the FEN to recover the diagrams...
See https://www.chess.com/forum/view/general/will-computers-ever-solve-chess?page=173 -- the two diagrams on that page do not display properly. They display just fine in V2, though.
[Edited later: never mind the above llink; I fixed these diagrams for Ponz. But another example is below, in post #6]
Environment: latest Safari 10.1.2 and OS X 10.12.6 with all updates.