What I find can happen with the contrast detect AF systems is they "look through" the subject to focus on what is behind if it has more contrast than the subject or if the subject is a little small. Generally though I look for two things focus acquisition and the subject becoming more defined or snapping to focus. The minimum focus distance is measured from the sensor plane and as stated above you are focusing on a virtual image, which is closer. You can use this to Visualise how far away you can focus: https://oceanity.com.au/tools/virtual-image-calculator/
For a 6 inch dome (with a 3" radius of curvature ) at 20cm object distance from dome the virtual image is 8.1cm from the dome, which is as low as the calculator goes. I would estimate the virtual image is 5cm from the dome with an object 10cm from the dome and it really should be able to focus on that. If you were focusing on coral maybe the back of the coral head with coral branches against clear water had the better contrast and it focused there. Just because it locked focus does not mean it focused on what you wanted to focus on.
Based on my Oly 12-40 which has the same more or less min focus distance you should be able to focus maybe 5cm from the port surface. You could try assigning focus peaking to a function button and confirm it's peaking around the thing you want to focus upon?
- Davide DB likes this