Thank you for the proper description!
You have stumbled upon a very rare, known bug, which affects the current release. Basically if only custom fields are selected as a search source, the search sometimes does not trigger correctly. I've already identified this issue and will roll out a release after the holydays.
As a temporary fix, I have enabled the “Search in permalinks” option on the General Options -> Sources panel, to trigger the search correctly and search the selected custom fields as well. I've choosen this option, because it has the least effect on the expected results.Best,
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