It is known that in Talmudic learning we cannot infer without discernment from the words of the Gemara, an inference is not an hazardous extrapolation. You cannot ask a question based on an hazardous extrapolation. A kushia needs a strong basis. If there is not a strong basis for the kushia, there is no difficulty to understand the pshat. You cannot infer from the Gemara 48 b regarding other topics.
The Gemara you cited says that regarding Averot we reset the counter. You cannot infer nothing regarding property.
Rashi says that the convert is compared to a newborn because he will not be punished for his past behavior:
אינו נענש על העבר
A good illustration of the fact that to say that the convert is as a newborn is not an undifferentiated truth is the Machloket between Rabbi Yochanan and Resh Lakish in daf 62a.
היו לו בנים בהיותו עובד כוכבים ונתגייר ר' יוחנן אמר קיים פריה ורביה Rabbi Yochanan said that a convert who had children before his
conversion, he has already left the Mitsva of piria verivia.
וריש לקיש אמר לא קיים פריה ורביה Resh Lakish said he hasn't.
רבי יוחנן אמר קיים פריה ורביה דהא הוו ליה Rabbi Yochanan says that he has already discharged his obligation because he has children
וריש לקיש אמר לא קיים פריה ורביה גר שנתגייר כקטן שנולד דמי Resh Lakish says he hasn't because a convert after his conversion is
comparable to a newborn
ואזדו לטעמייהו דאיתמר היו לו בנים בהיותו עובד כוכבים ונתגייר רבי יוחנן אמר אין לו בכור לנחלה דהא הוה ליה ראשית אונו This controversy is
based on the same principle as an other controversy between RY and RL.
A convert who has sons before the conversion has not Bechor regarding
the special status of bechor for inheritance, because the has already first
born
וריש לקיש אמר יש לו בכור לנחלה גר שנתגייר כקטן שנולד דמי Resh Lakish says he hasn't because he is comparable to a newborn
וצריכא דאי אשמעינן בההיא קמייתא בההיא קאמר רבי יוחנן משום דמעיקרא נמי בני פריה ורביה נינהו But despite that the two controversies are
based on the same principle, there is a need to teach both. Because
without the second controversy, we could understand that the reason of
RY is that regarding the Mitsva of Piria verivia only he is not as a
newborn because this mitsva was already relevant when he was not Jew
(Rashi says that about Bene Noach, the Tora writes פרו ורבו, and
tosfot explains that the chalenge is that his children are named after
him, and in the same sugya in Bechorot 47a, the Gemara says that it is from the mitsvatic point of view, they have the mitsva לשבת יצרה)
אבל לענין נחלה דלאו בני נחלה נינהו אימא מודי ליה לריש לקיש But regarding the rules of inheritance, Bene Noach have not the rules of inheritance, so
he is as as a new man regarding the inheritance's rules (Rashi explains that
they have rules of inheritance, but different from our, e.g. daughters inherit
as sons, and first born son as second son)
ואי איתמר בהא בהא קאמר ריש לקיש אבל בההיא אימא מודה ליה לר' יוחנן צריכא and if the second controversy was taugth without the first, we
might think that RL agrees to RY that regarding Piria verivia the
convert is not as a new man
איתיביה ר' יוחנן לר"ל (מלכים ב כ, יב) בעת ההיא שלח בראדך בלאדן בן בלאדן מלך בבל וגו' Rabbi Yochanan gives a proof that there is a link
of affiliation between father and son for non Jew from a verse
א"ל בהיותן עובדי כוכבים אית להו חייס נתגיירו לית להו חייס RL says that despite that there is a link of affiliation before the conversion,
it lapses from the moment when he converts
From this Gemara we learn that from the second controversy in which RL argues that a convert is as a newborn regarding rules of inheritance, we cannot infer what he said in the second controversy regarding mitsvat Piria verivia.
So, there is no source to innovate that he loses his property.
However in Kiddushin 17b, the Gemara infers from the Mishna Demai 6.10 that a convert doesn't inherit his non Jew father Deorayta.
גר את העובד כוכבים אינו מדברי תורה אלא מדברי סופרים
The svara of the comparison to a newborn (it's my understanding) is that a man when he converts is not less attached to himself than before the conversion. When he converts he became detached from the attachment to the non Jew world. He detaches himself, but he remains himself. His body is the same body, the property is an extension of the body. On the contrary, when he converts he is more close to his true ego. The world of falsehood to which he was linked and that was keeping him far from the truth is detached from him. Now he is not an artificial man, but a man more genuine, attached to what a man is. It is similar to Teshuva, David Hamelech asked to G-d לב חדש ברא לי please create for me a new hearth.
Note: an interesting expression in Yevamot 23a:
אדרבה שפחה ועובדת כוכבים הוה ליה לרבות דאי מגיירה לדידיה נמי תפסי בה קדושין לכי מגיירה גופא אחרינא היא
On the contrary, they needs to add a slave and a non Jewish woman, because if she converts, he can marry her by Kiddushin, the Gemara answer: If she convert, she is an other body.
