I am trying to make use of search synonyms. I have created a synonym group, and added synonyms comma separated, and afterwards ran full re index.
Now I am testing this out with the following API call
http://website/rest/V1/products?searchCriteria[filter_groups][0][filters][0][field]=name&searchCriteria[filter_groups][0][filters][0][value]=%25cucumber%25&searchCriteria[filter_groups][0][filters][0][condition_type]=like&searchCriteria[pageSize]=5&searchCriteria[currentPage]=1
This is returning data. But if I replace 'cucumber' with the synonyms , if returns no results.
I am using open search as my search engine, and I found this block of code in vendor/magento/module-open-search/etc/search_engine.xml
<engines xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Search/etc/search_engine.xsd">
<engine name="opensearch">
<feature name="synonyms" support="true" />
</engine>
</engines>
I am not sure if I am missing anything or doing it wrong.

As I couldn't find any article online which tells me why search criteria api doesn't work with synonyms, did my workaround. Sharing if this helps anyone in future. Created my custom search api.
Called the above function in search api, and added the synonyms as OR conditions
I have not tested this on different scenarios, but this works for my case.