Mirrors in Basement Glitch

Thanks for replying crinrict​  ​ I don't use mods or cc. I tried to add a video (it won't let me?), and then a gif (file size too big), and then screenshots (the issue doesn't appear in them). I've tried SO many ways now of trying to upload an image I just took with my phone, but I can't figure it out, even after visiting the specific error message's EA help forum (invalid HTML stuff)...sorry.

It's like the stairs are having a seizure; they are flashing rapidly between two positions---the one I placed them in and backwards. Thanks for the help; I know it's not much to go on, and an image would be helpful. I'm at my wits' end with this new challenge of image uploading I've discovered.

Edit: I should mention the flashing is happening with the "subtle light saucer" ceiling light too, although there's a chandelier that's fine, and any other wall/floor/table light sources are also not causing an issue.