In the edit mode of the description in the issue dialog, the "Save" button is not directly visible, if the description reaches a certain length. Then you have to scroll to find it.

If you forget to save directly, the following could happen:
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I had made more extensive changes and had forgotten to save – I assume because the button was not visible in that moment.
The next time I opened the issue, the editor was open and my changes were there.
Assuming that I had just opened the editor by mistake – as it works simply by clicking in the text – but had not changed anything, I instinctively clicked on "Cancel" and all my changes were gone.