I dove a bit in Redis and think I might understand something now.
It seems that Redis stores all simple values in string type format (or at least it doesn’t store any boolean values as boolean), so when you try to store a boolean value to it, that value must be type converted to a string value. php translates false to an empty string and true to a string with the number 1 in it, so that might explain the empty string return value of the get.
I have checked the Redis cache and it contained the key wp:tribe-events:tec_editor_compatibility_toggle_blocks_editor with an empty string value, so it is found by Redis and it will not return null or true/false as result of it. Instead it will always return the string value of the initial false that is stored as an empty string, and TEC doesn’t translate that to the falsey value of false.
When I remove the key and refresh the page the key is added again with again an empty value.
So redis will find always find the key (and not return false or null), but with a value that is of no use to TEC, since TEC always checks for boolean or null to determine what to do.
I am not sure where the value exactly is added, since I haven’t seen a call to the redis::set() method being executed. But from what I understand of the code in TEC it must be a type translation problem that causes it.
It could probably be solved by TEC by not trying to storing a settings in boolean format in the cache.
Also it seems that TEC stores the value in redis independent on what page I am. Refreshing a Redis settings page also added the key back to the redis cache. I would have expected that to happen only on an event admin page.
Well that must be enough for this error I think. The problem is clear, now TEC needs to decide if the plugin will change as a result. For now TEC and Redis are incompatible
