Linewise ^A, ^X behaves quite strangely

[ccjmne]

Reproduction:

docker run --rm -it archlinux sh
pacman -Syu --noconfirm vim git
mkdir -p ~/.vim/pack/tpope/start
cd ~/.vim/pack/tpope/start
git clone https://tpope.io/vim/speeddating.git
vim - <<-EOF
	top: 0
	right: 0
	bottom: 0
	left: 0

	a: 0
	a: 0
	a: 0
	a: 0

	a 0
	a 0
	a 0
	a 0

	0
	0
	0
	0
EOF

• On the top-right-bottom-left block, run vip<C-A>, to no effect;
• on the a: 0 block, vip<C-A> to have each line become b: 0,'
  • despite nrformats containing only bin,hex, and
  • neither a nor b being valid roman numerals—I would understand c become ci;
• on the a 0 block, vip27<C-A> to have each line become b 0,
  • it seems that we looped back over from z to a, which isn't how alpha nf behaves;
• on the 0 block, vip<C-A> does yield 1 on each line.

I'm sure I'm missing something, but I really can't put my finger on what exactly...

In case it's of any help, note that the g-prefixed alternatives, which I believe aren't mapped by speeddating.vim, behave properly in those cases:

top: 1
right: 2
bottom: 3
left: 4

a: 1
a: 2
a: 3
a: 4

a 27
a 54
a 81
a 108

1
1
1
1