It is an issue with the reagents. No need to send it in, they are sending out replacements. I just received an email a few hours ago to fill out this form to get new reagents:
https://pages.hannainst.com/hi783-reagent-issue?utm_campaign=HI783-HI784-Checker-Launch-2022
In the meantime I have been getting pretty good results using 5ml of reagent A.
Hopefully the new reagents are accurate. If they are then I think this will be a pretty good checker.
Edit - Here is the email I received from Hanna:
"Some of our customers reported issues when using our Marine Magnesium Checker® (HI783). We at Hanna, took a deep dive into this problem. We gathered customer feedback and performed extensive testing in our laboratory.
After investigating, we determined that our reagents used in the Marine Magnesium Checker® test were the root cause of this issue. We discovered that one of the chemical raw materials in the reagents has an unexpected degradation over time. This caused customers to experience high readings.
We will send new reagents to customers that purchased a Marine Magnesium Checker® (HI783). The estimated ship date is 6-8 weeks due to production requirements.
To our customers both near and far, we greatly appreciate your patience and understanding during this time. We apologize for any inconvenience that this has caused. We look forward to resolving this matter for you so that you can get back to what matters most, your reef tanks."