T0 belt slipping/mechanical problem? (IDEX)

For the last two weeks, I’ve been trying to fix resonance issues on my T0 printhead. At first, I thought the problem was caused by unequal belt tension. However, after printing the GATS test and re-tensioning the belts, the resonance was still present. The graphs shown were taken with a 3.2 kg weight (I also tested with 2 kg, with similar results).

As a next step, I disassembled the motors on the left side that drive the T0 axis, and reassembled them while carefully checking each pulley to ensure smooth rotation (including the gantry pulleys). After reassembly and belt re-tensioning, I noticed that the belts for T0 were moving up and down significantly (see video).

For now, I skipped the input shaper results and tried printing directly, but T0 experienced severe layer shifts, most likely due to belt slipping.

Does anyone know what could be causing this belt slipping and the poor shaper results?