It is possible the MAMP PRO installer did not copy all of your database data successfully.
Try the following ...
Your old MySQL 5.6 data is in
/Library/application support/appsolute/MAMP PRO/db/mysql56_2018_xxxx
To be on the safe side, first make backups of your host data, and your entire /Library/application support/appsolute/MAMP PRO/db/
To verify this is your old data, look inside of this mysql56_2018_xxx folder, you should see subdirectories of all of your old databases in this folder.
If you see missing databases in MAMP PRO 5, you can "reupgrade" your old data by doing the following.
1. Quit MAMP PRO
2. Rename your mysql57 folder to mysql57_back
3. Make a copy of your mysql56_2018_xxxx folder, name it mysql57.
4. Start MAMP PRO, start your servers
5. Run Tools > Upgrade MySQL DB