This is explained inthe course notes for the section on 'Numbers'. The numbers pump, chwech and cant each lose their last letter when they are used in front of nouns, adjectives and other numbers.
As rmcode says, it is a bit of an oddity that is sometimes ignored in the wild, especially before vowels.
If you ask in the Duolingo support forums somebody there may well have an answer - https://support.duolingo.com/hc/en-us/articles/204762054-Troubleshooting
The course teams do not have any access to the inner workings of the system, I am afraid.