Mining blocks with the Chainsaw causes them to drop both their normal drops from being mined with a regular tool, AND their special drops from being mined with shears. The only block I've found in vanilla that this affects is leaves, but I'm sure there are modded blocks where this is a much bigger issue.

The Chainsaw should only make blocks generate one type of drops, not both.

I'm using IC2Classic-1.12-1.5.8.jar and 1.12.2-forge-14.23.5.2859.jar.

That is actually a vanilla mechanic iirc.

No, it's not vanilla. When you mine Leaves with Shears, they never drop Saplings. I just tested it myself, and the wiki also says so:

Leaves that decay, or are destroyed without using Silk Touch or shears, yield saplings 5% (1⁄20) of the time, sticks 2% of the time, and otherwise drop nothing.

yeah it was 3AM when i wrote that.
Still it's intended behavior.