This is because substr prints the number of characters after the starting index. Had you started from 0, you would just need to subtract 1 to exclude the last element.
But since you have started from 1, it has shifted the range by 1, which means the last character is included again. So you need to subtract 2.

According to documentation, the last index is the index of the first character to exclude, rather than the last one to include

Have you gotten an answer yet?

I also did -2 instead of -1 and it worked lol