With ITT 2.1.5 and OpenBlocks 1.4.2, right clicking on a OpenBlocks tank with an ITT clay bucket removes one bucket of lava from the tank, but does not give you a lava-filled clay bucket. You retain the empty clay bucket. Any ideas?

Encountering this in my modpack. I'm not aware of anything that should prevent the bucket from working.

I do know that if you use a clay bucket to place lava, the bucket is consumed once the lava is placed.

I just tested your situation in SkyFactory 2.5 which has ITT 2.1.6, and I get a clay bucket filled with lava. I suggest that you update your copy of ITT in your pack, and as such mark this as closed as this is not an issue in the current release.