This was not a "normal" dispute in Sevara but a simple question of fact as to what R. Yochanan said.

See Rashi (Kesuvos 57a DH ha'KM"L) that where two Amora'im argue about what was said, one of them is "Meshaker". What is meant is that one got it wrong. See Tosfos there that one "Ta'ah b'Shmu'aso". See also Nimukei Yosef Yevamos 77a (25A b'Dapei ha'Rif) b'Shem Ritva d'Omer bede'Dami Lei.

See in great detail Yad Malachi s.663 especially p.170 where he rejects the view of Halichos Olam [that the Rebbi changed his mind which was known to only one of them] for our Gemara proves that the Amora'im themselves were prepared to state categorically that one of them had simply got it wrong. He also quotes Shevuos 26a.

On p.170b Yad Malachi continues to explain that they were not accusing each other of Sheker b'Mezid but of lack of precision in their attention and rendition, Ayin Sham.

I would add on a historical note that Rav Dimi had worked very hard indeed to bring Toras Eretz Yisrael to Bavel and vice versa and though he had made mistakes in the past (e.g. Shabbos 63b) he was sure of himself on this occasion and therefore used very strong language to emphasize that one or other of them had got it completely wrong.

Best regards Mendy!

Kol Tuv,

Joseph Pearlman