Keeping it simple the /effect command used to give status effects is not working how it should. When using the command as shown /effect @a[tag=fairy] regeneration 15 1 true it should give the player with the tag of fairy regeneration for 15 seconds with an amplifier of 1 which is the normal rate for regenerating the issue is instead of gradually filling the health bar it nearly instantly fills it making anyone who has the effect actively unkillable further if you change the 15 to anything below 5 the effect appears active but doesn't actually heal you even if in a repeating command block. long story short anything above 5 seconds gives a rapid regen while anything below gives no actual regeneration but the status effect appears active regardless of amplifier. This issue also appears with other status effects but it's not as noticeable as with regeneration because you can visibly see the way the timer and amplifier effect the health bar.   

Sorry I am not the best with grammar and punctuation.