Multiple Series (Combine two questions)

Just tried a different browser, not a caching issue.
The strangest thing is that I can add some questions, but when starting from those same questions I can't add the other. So A can add B, but B can't add A.
A doesn't show in the list of questions available to add.

Here's the Diagnostic Info:

{
"browser-info": {
"language": "en-US",
"platform": "MacIntel",
"userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.149 Safari/537.36",
"vendor": "Google Inc."
},
"system-info": {
"java.runtime.name": "OpenJDK Runtime Environment",
"java.runtime.version": "11.0.5+10",
"java.vendor": "AdoptOpenJDK",
"java.vendor.url": "https://adoptopenjdk.net/",
"java.version": "11.0.5",
"java.vm.name": "OpenJDK 64-Bit Server VM",
"java.vm.version": "11.0.5+10",
"os.name": "Linux",
"os.version": "4.15.0-1052-azure",
"user.language": "en",
"user.timezone": "UTC"
},
"metabase-info": {
"databases": [
"postgres"
],
"hosting-env": "unknown",
"application-database": "postgres",
"application-database-details": {
"database": {
"name": "PostgreSQL",
"version": "10.11"
},
"jdbc-driver": {
"name": "PostgreSQL JDBC Driver",
"version": "42.2.8"
}
},
"run-mode": "prod",
"version": {
"date": "2020-02-25",
"tag": "v0.34.3",
"branch": "release-0.34.x",
"hash": "1a83edb"
},
"settings": {
"report-timezone": "Canada/Eastern"
}
}
}