After #2934 was fixed, most problems with singletons seem to be solved. There still is one left. I don't know if there's an easy way to fix this - if not, I'll add an exclusion rule in my code.

I have the following record class where SpotBugs still reports a false positive SING_SINGLETON_HAS_NONPRIVATE_CONSTRUCTOR:

    record RowInfo(int firstRow, int lastRow) {
        private static final RowInfo NONE = new RowInfo(0, -1);

        public static RowInfo none() {
            return NONE;
        }
    }

The problem I think is that it's not easy to tell if something should be a singleton or not. And you could implement a singleton as a record, but it is rather usual IMHO. I this example, the static final member is not intended as a singleton instance, but is used as a guard value.