About not able to restore see this release notes: https://docs.mendix.com/releasenotes/developer-portal/mendix-cloud
Mendix Cloud database backups are now created with pg_dump version 1.14. This version is shipped with PostgreSQL since October 2019 (PostgreSQL 12, 13). The side-effect is that it is not possible to restore these PostgreSQL backups using a pg_restore version below 1.14 (PostgreSQL <= 11). The error that you will receive is pg_restore: [archiver] unsupported version (1.14) in file header. To resolve this issue, upgrade your PostgreSQL client software to a version that includes newer versions of pg_dump and pg_restore, for example PostgreSQL 12 or 13.
So try it first with a newer PGAdmin. If you keep those warnings I would suggest filing a support ticket.