Went to test this and I cannot save a relationship value.
I installed a clean version of 4.9.8 and also 5.0 to test on with the same results. The database structure was created properly and happy to see the extra columns. Here are the steps I took to test.
- Installed Metabox from wp.org repo.
- Installed MB Relationships from Github repo.
- Create plugin to instantiate relationship field. Copied code from documentation.
- Created 1 page and 1 post.
- Set relationship with Post Object select field.
- Select Publish.
On save, the relationship fields get cleared out. Reset back to a single empty value.
Am I missing something on my setup? Have you tested on a fresh install?