Welcome @axiumone !
Actually the height of the standoff is not the challenge. The OBT mini and the OBT are designed to withstand full Extreme Overclocking, with massive super heavy cooler. Your d15 is actually a lightweight compared to specifications the OBT was built for.
The mini respect the ATX specifications and if your motherboard bows there, it would in any other cases
The reason your board bow can be :
- strong or too tight cooling system
- strong or too tight CPU Cooler backplate fixation
- Limited rigidity of the motherboard PCB
Please post pictures so that can help for diagnosis !