MacBook Pro 13" (2018) cannot drive 4k display @ 60Hz from right-hand USB-C ports
In 2016 and 2017, the left-hand Thunderbolt ports were higher bandwidth than the right-hand ones, and this would be expected behaviour. For 2018 models, all 4 ports should have the same performance.

So is it possible that you have a 2017 model without knowing it? If not, then I think this is unexpected behaviour.