The project requires version ^3.1.0 of the vlucas/phpdotenv package which is unsupported by what appears to be the unmaintained philippbaschke/acf-pro-installer.
This needs to be investigated more. Feels like Pantheons composer example as lagging behind Bedrock.
Solution
The short term solution is to downgrade the vlucas/phpdotenv version to ^2.6. Bedrock currently relies on ^4.1.5 of vlucas/phpdotenv.