3.8:

import org.checkerframework.checker.nullness.qual.Nullable;

import java.util.Iterator;
import java.util.List;

abstract class YourClassNameHere {
  @Nullable Number foo(List<? extends Number> numbers) {
    return getOnlyElement(numbers);
  }
  
  abstract <T> @Nullable T getOnlyElement(Iterable<T> values);
}

3.7: nullness verification OK
3.8:

Error: [type.argument.type.incompatible] incompatible type argument for type parameter T extends Object of getOnlyElement.
   found   : ?[ extends @UnknownKeyFor Number super @KeyForBottom Void]
   required: [extends @UnknownKeyFor Object super @UnknownKeyFor null]

The error appears when trying to upgrade the Checker Framework from 3.7 to 3.8 in Apache Calcite.

CI: https://github.com/apache/calcite/pull/2294/checks?check_run_id=1531065596#step:4:263
PR: https://github.com/apache/calcite/pull/2294

It looks like the workaround is to declare getOnlyElement as getOnlyElement(Iterable<? extends T> values), however, Guava's signature is Iterables.getOnlyElement(Iterable<T> values)

Do you think it is CF issue?

Start by reproducing the Java snippet with Checker Framework 3.7 and 3.8, then inspect the linked Apache Calcite CI run and pull request. Trace the generic type-argument and nullness-checking diagnostics to determine whether the 3.8 behavior is a regression. Done means the behavior is explained and either corrected or documented with a confirmed workaround.