Creating a montage from a default frame rate and rate scale sequence reports 1 less frame than the sequence. The difference is a result of UAnimSequence overriding UAnimSequenceBase::GetNumberOfFrames. As frame rate is calculated by (frames-1)/length, then GetNumberOfFrames should be modified to return (rate*length)+1.
Browse to an animation asset with a frame rate equal to the default sample rate(30) and a rate scale of 1 in the content browser.
Hovering over it, notice its Number of Keys.
Opening it, notice the divisor of its 'CurrentFrame: # / # Frame' is Number of Keys -1.
Create a new montage from the sequence and open it.
Notice the divisor of its 'CurrentFrame: # / # Frame' is Number of Keys -2.
Expected: A montage created from a sequence with the same frame rate and default rate scale should report the same number of frames.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-80077 in the post.