Thanks for raising this issue, I have learned a lot about OBC (Optimised Battery Charging) and OBL (Optimised Battery Limit) which I didn’t realise were separate things until seeing
Apple’s article about OBC and OBL which says this:
“With Optimised Battery Charging, your Apple Watch learns from your daily charging habits to improve the lifespan of your battery. And supported Apple Watch models offer additional battery charge optimisations with Optimised Charge Limit”.
My Ultra 3 on latest watchOS27 beta has no setting for OBC, only OBL, so it looks as though OBL supersedes OBC on supported models. On the watch screen in Battery settings the small writings says:
"When Optimised Charge Limit is turned off, Apple Watch may still wait to finish charging past 80% until you need to use it".
.....That sounds like on watches that support OBL, you can't turn off OBC.
The Apple article also has this:
"Optimised Charge Limit changes dynamically depending on how you use your watch. Based on your usage patterns, your watch may charge to 100% on some days, and less on others".
...which is a bit surprising. If it wasnt for this I would have assumed OBL was like the limit setting on phone and Mac.
I have OBL turned on and have never seen 80%. I have also never seen the open charge ring you mentioned and shown in the article:
View attachment 2649355
....but it is possible I have never noticed it! Unfortunately my watch has only just been charged to 100% and if I put it charger it does not look like the screenshot. I can’t check if shows it when I put in charge when it is less than 80%.
The watch still has to learn your usage with OBL so your situation may be because Apple may have tweaked the algorithm in last beta, or it may be a bug or it may be re-learning your usage.
I haven't had my Ultra 3 very long and it has only been on latest beta and the one before, but it sounds like OBL is misbehaving for both of us in different ways.