forge-1.12.2-14.23.4.2739-universal.jar
minecolonies-1.12.2-0.9.37-ALPHA-universal.jar

Expected behavior

Set the backpack (for example mining backpack) in receiving mode. Shift-click a rack with ores inside. Ores are transfered in the backpack.

Actual behaviour

Shift-clicking a rack switches the backpack mode as if it was done in the air.

This is happening since a few releases ago.

I don't remember exactly. It began with the new rack system, probably at the same time of the new file naming scheme (alpha-beta-release).

Please give us version numbers

I checked into this,can you report this to the backpack mod, the issue shouldn't be on our side.