Bugs in Marketplace content usually occur due to changes in updates that alter the way certain behaviours and machines act, something that cannot always be anticipated by creators.
If you find an issue with a piece of Marketplace content:

The Marketplace creators are all independent companies, which means that they each have different ways of getting in touch! Most of the teams have their own dedicated websites or social media accounts.
It’s important to contact the creator first, as they’re often the first to know if something is amiss with their content and may already have found a solution.

In the event that the creators themselves can’t help you fix the problem, the best thing to do is to contact Minecraft support for further assistance. This might happen if the bug is unusual and the normal troubleshooting methods aren’t working, or the problem is not with the content itself.
When contacting anyone about a Minecraft bug, remember to offer as much information about the issue as you can; this includes telling them what device you’re using, which map you’re having a problem with, the Minecraft version number (found at the bottom of Minecraft’s main menu) and precise information about what has gone wrong (including pictures if applicable). This will help the team diagnose the problem and find a solution for you quicker!