/execute at @p run effect give @e[type= !minecraft:player] minecraft:glowing 1 1 true on repeating command block. even after the command block is destroyed, all entities still glow forever. Trying to remove glowing with this with commands does not remove the effect. the only way of doing it is by killing all mobs. Before 21w15a the command worked just fine and the glowing stopped after the command block was destroyed.