If so the problem is with the stick, box or the TV.
I did find someone with a 50 foot HDMI cable and that was their issue.
When we plug specific devices into our video wall, 3/4 of the screens turn black - but only after a delay of a few seconds, leaving only the top-left screen turned on and showing content. The video of this happening is available here:
This happens with a Fire TV stick and a Roku TV box, but not with a laptop connected directly.
It's worth noting that we have a video switcher before the TVs which allows switching between different outputs - so this could have some effect on it, but we've done some testing and if we plug the Fire TV stick or Roku TV box (the two devices which are exerting broken behaviour) into the port where the laptop plugs in directly through HDMI, then they still don't work. The switcher we are using is this one: https://www.kvmsolutions.uk/product/sy-uhd-s41a-s-y-ellectronics-4-port-hdmi-uhd-switch/
After some investigation it doesn't appear to be a problem with the switcher, as the behaviour is roughly the same when bypassing the switcher completely and plugging directly to the TVs