When I right click on a horse with empty hands nothing happens. I am only able to test this on untamed horses/donkeys, but have tried it on a freshly spawned horse as well as two naturally spawned donkeys with the same result.

Right clicking with an item in my hands causes the horse to rear/neigh. Mounting boats still works.

I can reproduce this after server restart on my local machine. Can reproduce after changi gamemode to creative. Can reproduce with different key mapped to 'use/place'

cannot reproduce in single player game. Cannot reproduce on new server hosted in the same way with the same settings.

CAN reproduce in new server by replacing new world files with world files from original server.

Cannot reproduce when deleting player data from server. Attaching problematic player data for review, since deleting/reforming that data resolved the issue.