[chaley]

There was a change on 19/Feb to strip trailing zeros from series indices. The change appeared in calibre V5.38. I strongly doubt that it will be changed back.

You have three choices:

1. Revert back to calibre 5.37 or earlier. You will be stuck there.
2. Rename the existing books to get rid of the trailing zero.
3. Change to this template that formats the series_index differently depending on whether it is a float or an integer

4. {author_sort}/{series}/{series_index:'if floor($) != $ then format_number($, '0.2f') else $ fi'} {title} - {authors}