Yes, some people find that SSRIs like citalopram can blunt the effects of psilocybin, sometimes even after stopping. Experiences vary a lot, from noticeably reduced visuals or emotional depth to almost no effect, while others report no change at all.
A common explanation is that long term SSRI use may reduce sensitivity at serotonin 5-HT2A receptors, which psilocybin targets. Evidence is mixed, and I cannot verify how long this might last for any individual.
Reports suggest the dampening can persist for weeks or months after discontinuation, then gradually fade, but there is no reliable timeline. For a practical overview of SSRI use and psilocybin truffles in trip therapy, see this discussion: SSRI and truffle interactions.
I hope this clarifies what people commonly report.