In developing my modpack I discovered that despite having TombManyGraves 2 (v4.2.0), TombManyPlugins (v1.0), and TombManyPlugins - Wearable Backpacks (v1.0) installed alongside Wearable Backpacks (v3.1.3) the advertised interaction did not seem to function. To verify that this wasn't a mod interaction with some other mod in my modpack, I made a new instance with the following details:

Minecraft v1.12.2
Forge-14.23.5.2795 (latest at this time)
Just Enough Items v4.13.1.225
Tomb Many Graves 2 v4.2.0
TombManyGraves 2 API v1.0.0
TombManyPlugins - Wearable Backpacks v1.0.0
Wearable Backpacks v3.1.3

I tested this by equipping the backback, putting an item in it, and then jumping to my death. First I tried with the Wearable Backpacks config option to place the backpack on the ground as a block, and then with the Wearable Backpacks config option to drop the backpack and its contents on the ground as items. In both cases, dying with the backpack on did not result in the backpack going into the gravestone and the Wearable Backpacks config option took precedent.

Do note, as of 2.4.0, the API and addons are all included in the main mod again, so you probably shouldn't be using the addons anyway.
That said, I can confirm this is also an issue WITHOUT the outdated API/Addon, in version 2.4.0. Same issue, same results. The backpack just doesn't go into the grave at all.