If you have an existing transaction entry, either split or not, if you go to the split screen, the 'show %' radio button is there. The percentage column is there as well, but not really present until you check the box.
In a new transaction, if you go to the split screen, there is no 'show %' check box, but the column is present on the right-hand side.
This has to be some sort of bug.
Quicken Business & Personal, version R69.53, build 27.1.69.53 on Win 11.