Every menu option rendered as selected. Strange workaround - react-select

Question

I just upgraded two projects from using react-select 1.2.1 to 2.0.0. The first project is a "demo" for trying out react-select, a simple one based on Create React App. The other project is the serious stuff, including old and new technologies (Java, Maven). In the simple project the new react-select worked as predicted, but in the serious project I found that when I had made a selection and opened the menu again, all the options were rendered as selected!
I then found out about the isOptionSelected property and I tried this in my simple project first:
isOptionSelected={(selOpt, selOptArr) => false}
As expected, this caused no option to be rendered as selected. However, when I put the same line into the serious project, it cured the problem there; now only the correct option was rendered as selected!
This is the complete Select definition:
<Select
components={{ DropdownIndicator }}
styles={mySelectStyles}
placeholder={someText}
isClearable={true}
isDisabled={readOnly}
isOptionSelected={(selOpt, selOptArr) => false}
noOptionsMessage={() => "No match"}
value={value}
onChange={(selectedItem) => {
if (selectedItem !== null) console.log(...);
}}
options={myOptions}
/>
I have not been able to find out why the line with isOptionSelected causes different rendering in my two projects. To me it seems like a bug in react-select 2.0.0.