The most recent version of Better Together:
Tamed horses transferred via sync to the new version (then added to a Realm) won't enter love mode/won't breed. The horses had saddles, and some with armor.
-Fed them tons of wheat, so they were not 'hungry,' they would eat the wheat infinitely
-Fed then two golden carrots, no love mode
-They would just keep eating the golden carrots infinitely
-Never entered love mode/wouldn't breed
Edit by: [MCPE Mod] KillerbeeNL
Converted horses donkeys and mule don't have a health-bar (hearts) most likely causing the issue.
Converted horse / donkey:
Newly spawned horse / donkey: