Can’t provide a screenshot owing to the nature of the bug, but sometimes when attempting to purchase a ring slot with plague ichor, the ichor will be spent but the slot will not be acquired. The slot continues to display the ‘locked’ appearance in the Watcher’s UI for Plague Ichor. On leaving that UI, the slot will also display the ‘locked’ appearance.
I acquired another Ichor and attempted to purchase the slot again. I was met with the same appearance of an un-acquired slot in the Watcher’s buying UI, however, when I later looked again at my inventory UI after leaving the Watcher, this time the ring slot was actually unlocked.
The only other difference to draw out is that in my first attempt, I’d clicked through the UI dialogs for purchasing the slot with an ichor as quickly as I could; whereas in the second attempt, I applied pauses between the UI dialogs for purchasing the slot.
Other users on the Discord had reported encountering this problem as well when I asked about experience with this there.
Edit: Link to logs