Don't have this issue on my S20+. I assume you already tried reinstalling Instagram, does this occur on other apps like Facebook Stories? Try loading up an HDR content on YouTube and see if it flickers there if not, maybe you have to wipe everything that is Instagram related in the root folder.
re installed Instagram. No success... same story looks proper from one plus 7t and also from s6 edge plus. Only S20PLUS has the issue... Tried contacting Samsung for this issue. They told to contact Instagram developers !!!
I just bought s20 plus 4 days ago and in instagram stories it is flickering annoyingly lot. It seems google pixel had this issue too but google fixed it for their products. Samaung ahould fix this asap.