The official docs on resource name constraints do not list the managed environment only the container app itself.
Notably there is a GitHub ticket requesting for the constrictive limit to be increased:
Feature Request: Allow container app names to have length > 32 which makes me mindful that the environment itself has a limit.
I'm keen to know the exact limit as I am building templates which also need to adhere to internal naming conventions (based on the Azure Cloud Adoption Framework), so the names tend to be quite long.
(I know the answer now, so will post directly afterwards, for community information).