Thanks for the save, it helped me to figure out what was causing it, this one was quite hard, but the more happy I was when I solved it :)
I used variant of it for the automated test that checks that splitters won't uncompress belts this way again.

TL;DR
Fixed for the next release.

I have ...

Can you reproduce this using only two rows of splitters? (Having only one splitter-to-splitter feed per lane)

This is the simplest setup I could reproduce the bug:
f_gaps2.png

More gaps with more splitters:
f_gaps.png
More visible when it is moving...

Note: sometimes the gaps disappear ...

Can you reproduce this using only two rows of splitters? (Having only one splitter-to-splitter feed per lane)

This is the simplest setup I could reproduce the bug:
f_gaps2.png

More gaps with more splitters:
f_gaps.png
More visible when it is moving...

Note: sometimes the gaps disappear ...