I was able to repeatably reproduce this and I’ve found that it’s entirely due to Gboard’s autocomplete system.
I’ve found two ways to replicate it:
- Type a word and have Gboard autocomplete it as the last word in a list
(a good example is the wordI'llbut without the') - Use a suggestion from above the Gboard keyboard as the last word in a list
I’ve not (yet) found any other ways to reproduce this.
As for ways around the problem, one way is of course to turn off auto-complete.
But that’s no fun 
.
Another is to notice when a word has been corrected, or when you click a word suggestion and make some other edit to that same line.
You could add a full stop, delete the inserted space, or any other text edit (I think).
Personally I think I’ll either try to go for full stopped lists (yuck) or try to notice when there’s a space at the end of a list and delete that before hitting enter.
Watch out for Gboard correcting the last word in your bullet list!