No, it doesn't occur for me. I have 9 mouth shapes in the same order as Rhubarb Lip Sync mouth shapes. A-H, X.
A = 1, B = 2, C = 3, D = 4, E = 5, F = 6, G = 7, H = 8, X = 9, where X is the idle mouth shape. There's no need to create a mouth shape for x as you can just insert the first frame of your idle character animation for the same direction.
The animation should be played forwards. Should not have the set pause for each frame option ticked. I don't think the pause value matters. The first & last value in the tsv file seems to get ignored. My recommendation is to not mess with the tsv files & just insert an extra frame as frame 8 where the H mouth shape should be so that it correctly uses frame 9 which is the X mouth shape.
I had a mess around earlier & it seems that if you swap out mouth shapes for frame numbers in the tsv file that it starts from 0 instead of 1, so frame 1 is 0 instead of 1 & frame 9 would be 8 instead of 9 - I don't know why.