When used with First Loop Start Frame Offset and Use Animation Blueprint, the character will alternate between the specified animation and the reference pose

    AnimPosition += InFrameRate.AsSeconds(FirstLoopStartFrameOffset);
   if (SeqLength > 0.0 && (bLooping || !FMath::IsNearlyEqual(AnimPosition, SeqLength, 1e-4)))
   {
      AnimPosition = FMath::Fmod(AnimPosition, SeqLength);
   }
   AnimPosition += InFrameRate.AsSeconds(StartFrameOffset);
   if (bReverse)
   {
      AnimPosition = GetSequenceLength() - AnimPosition;
   }
 
   return AnimPosition;
} 

    // The Time from the beginning of SectionStartTime to InPosition in seconds
   double SecondsFromSectionStart = FFrameTime::FromDecimal((InPosition - SectionStartTime).AsDecimal() * AnimPlayRate) / InFrameRate;
 
   // Logic for reversed animation
   if (bReverse)
   {
      // Duration of this section 
      double SectionDuration = (((SectionEndTime - SectionStartTime) * AnimPlayRate) / InFrameRate);
      SecondsFromSectionStart = SectionDuration - SecondsFromSectionStart;
   }
 
   // Make sure Seconds is in range
   if (SeqLength > 0.0 && (bLooping || !FMath::IsNearlyEqual(SecondsFromSectionStart, SeqLength, 1e-4)))
   {
      SecondsFromSectionStart = FMath::Fmod(SecondsFromSectionStart, SeqLength);
   }
 
   // Add the StartFrameOffset and FirstLoopStartFrameOffset to the current seconds in the section to get the right animation frame
   SecondsFromSectionStart += InFrameRate.AsSeconds(StartFrameOffset) + InFrameRate.AsSeconds(FirstLoopStartFrameOffset);
   
   return SecondsFromSectionStart;
} 

double FMovieSceneSkeletalAnimationSectionTemplateParameters::MapTimeToAnimation(FFrameTime InPosition, FFrameRate InFrameRate) const
{
   ...
 
      //for fix
      SecondsFromSectionStart += InFrameRate.AsSeconds(FirstLoopStartFrameOffset);
   
   // Make sure Seconds is in range
   if (SeqLength > 0.0 && (bLooping || !FMath::IsNearlyEqual(SecondsFromSectionStart, SeqLength, 1e-4)))
   {
      SecondsFromSectionStart = FMath::Fmod(SecondsFromSectionStart, SeqLength);
   }
 
   // Add the StartFrameOffset and FirstLoopStartFrameOffset to the current seconds in the section to get the right animation frame
   // for fix
   //SecondsFromSectionStart += InFrameRate.AsSeconds(StartFrameOffset) + InFrameRate.AsSeconds(FirstLoopStartFrameOffset);
   SecondsFromSectionStart += InFrameRate.AsSeconds(StartFrameOffset);
   
   return SecondsFromSectionStart;
}