As already posted by TakingItCasual some 3 years ago, active filtering machines will stop filtering when loading a savegame. Only if a new filter is inserted will they finish using the one that was in the machine when the game was saved. In always-on-setups with multiple filters available to a machine, this will cause no problems, but if you use the machines with only one filter each (in my case to be controlled by a pollution sensor, to do one cycle whenever a certain pollution threshold is reached), one has to manually add/remove one additional filter to each machine that was active when last saving the game. For large factories this a terrible pain... Maybe this could be fixed? Would be a shame if not, i really like your mod otherwise!

check out this blueprint to know what i mean (requires https://mods.factorio.com/mod/pollution-detector):

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