Too much pressure on AC joint while using yoga headstand/inversion bench?

I recently got a [yoga headstand bench](https://www.theyoganomads.com/yoga-stools/), and have enjoyed using it so far. Earlier this week, however, I felt some discomfort in my left AC joint (shoulder) after using it. I possibly wasn't supporting myself enough with my arms and left too much weight where my shoulders meet the bench. In any case it was a mild discomfort so I shook it off and felt normal the rest of the day.

The next morning the same shoulder was a bit sore, but I assumed I had just slept on it weird. Then during the first downward dog of my morning practice I felt a sudden hot pain in my left AC joint. I have reduced mobility and am pretty sure I sprained it. At the time I stopped and haven't put pressure or over extended the joint since then, and it's definitely getting better. But I'd really like to avoid this in the future.

Has anyone else experienced this? Am I missing something about proper form on a headstand bench? Do folks frown upon their use in general? I guess they are a bit of crutch.