I've noticed while having cholera or food poisoning (unknown food can), but may also apply to other sicknesses, and curing heatstroke, causes the sickness indicator in the UI to disappear despite still being sick with cholera.
This occurred for my friend after eating an unknown food can durring our session and after curing heatstroke, continued to vomit with no indicator.
WORKAROUND:
Relogging confirms the character is still sick as the sickness icon returns and (what I can only assume is an indicator to the player they are still stick) vomiting upon spawning into the map.

• Contract sickness, such as Cholera or Food Poisoning (may also apply to other sicknesses)

• Confirm the sickness icon has appeared in the UI

• Contract heatstroke (blurred vision (may also contribute to vomiting?))

• Cure heatstroke by avoiding sun, cooling off with water

• Sickness icon should disappear after curing heatstroke

• Confirm character is still sick by consuming a food / water item (full bottle of water, whole can of food, etc.)

WORKAROUND:

Can also confirm character is still sick by relogging. Will result in icon reappearing.

In the example of cholera I would expect the sickness indicator to remain present as consuming too much food / water will result in vomiting even after the indicator has disappeared.

Addition Thought:

Could be related to sickness that involves vomiting if heatstroke does indeed induce vomiting on the character. Just a spur of the moment thought that may have no merit.