Test a few things...
Choose MATCH MONITOR (not Match Content) for the sequence preview. That should be a good representation of the export.
On files with aspect ratios that don't seem to be working correctly, open effects and apply the Aspect Ratio/Letterbox option. Experiment with the choices in the drop-down box.
If this doesn't remedy the issue, please share your project. Someone will check it out. This is an easy 4-step process, and it can be private. Don't submit the export, just the project...
- With your project on the timeline, click the 3-lined 'hamburger' menu, top-left. Click FILE | BACK UP PROJECT FILES TO FOLDER. Specify a folder. Click SELECT FOLDER.
2. Using the PC file manager, go to where the numbered folder was saved and upload it to a free server - Google Drive, MS OneDrive, etc.*
3. Get a shared, public link that anyone can view.
4. Paste that link here, or click the folder, above top-right in this forum, and message it to me. This is private and won't be shared.
* Before uploading, right-click the folder, click PROPERTIES. Look at the File Size to confirm that it's not too big for the free space on the server.