today I’ve found this on GitHub repo. https://github.com/AdvancedCustomFields/acf/issues/567
The problem was already acknowledged. The interesting part is the temporary workaround: wrap acf block in a group and assign the anchor to that.
I’m experiencing the same issue! Googling around I found that this problem (saving anchor value due to incompatibility with a WP helper function) already arose and was solved in the past. See: https://github.com/elliotcondon/acf/issues/768