I'd guess more likely that some local in-browser state during your session "flipped" to indicate the target account of interest didn't exist after a failed API

I'd guess more likely that some local in-browser state during your session "flipped" to indicate the target account of interest didn't exist after a failed API interaction or a logic error. Your short term switch to an incognito tab created a new session without the flaw, making the target account visible. Your regular-tab interaction eventually refreshed state that made the target account appear. ... that's my attempt at a most plausible explanation, and doesn't involve any purposeful blacklisting of the target account on IG's part.