After testing similar setups in both 1.14.4 and 1.15.2, the mechanical saw was able to cut down an oak tree (single block diameter trunk) in the older version, but would only break the bottom block in the newer one.

The saw was placed too close to the block to be broken. Moving the setup an one block further from the tree resulted in the entire being cut down when it was pushed into it.

I am on the same server/modpack as poster and can also confirm the issue. Here is the modpack mod list: https://www.curseforge.com/minecraft/modpacks/ultimate-pack-1-15-2/relations/dependencies

We are not sure if this is a create mod issue or some other mod that is breaking the function of the mechanical saw.