You might want to reach out to the author of TSR_MapOverlay if the issue also occurs without [CGMZ] Light Effects, as it sounds like that plugin is most likely the issue if it is having the same problem with both [CGMZ] Light Effects and KC_Mirror.
Viewing post in [CGMZ] Light Effects for RPG Maker MZ comments
Ok, sorry I could not help much, but I need to reproduce the problem first before I can investigate what is happening. You could try some kind of preloader for your water layer graphic or whatever it is that blinks. It could just be that the plugin is struggling to load it in time when it also has to load mirror sprite + light sprites, I am not sure.