לע"נ ר' חיים צבי בן ר' אליהו
The sixth Ani Maamin in the siddur states:
אני מאמין באמונה שלימה שכל דברי נביאים אמת.
I believe with a complete faith that all the words of the prophets are true.
But the Rambam [on whom the Ani Maamin is ostensibly based doesn't say that. The Rambam says
In other words, we must believe that there are people who receive prophecy and not that all of their words are true. The Rambam also writes in Yesodei Hatorah [7/1] מיסודי הדת לידע שאל מנבא לבני אדם.
Once we believe that Hashem gives prophecy to people then of course we have to believe that their words are true - if he is really a prophet. That goes without saying.
[See Yeshurun 22 page 500-501 for more].