That is it. When TV's first started using Filmmaker mode it was a setting that was off by default and you can turn it on if you wanted it. For some reason, this has progressed to some manufactures automatically turning it on by default for movies that contain the metadata.
Even in the TV manual it states that movies will be darker when using Filmmaker mode. I honestly don't know anyone that prefers this setting as we are used to watching shows with the picture mode we have set it to use.
If your TV has a setting to automatically switch to Filmmaker mode, turn it off. Other TV's have a Picture Mode that you can use when it detects Filmmaker mode to override the darker picture you would normally get.