It likely would not have been as noticeable but for the juxtaposition of the two pesukim. In pasuk 64, ish is translated as gevar, while in pasuk 65, ish is translated as anash. Why the disparity?
Well, Onkelos is a person, rather than a machine, and human language is also not as precise as we might like, so some variance in language is possible, without it meaning much.
However, I did a search for all instances of the word ish, and selected those in sefer Bereshit. This because Onkelos' translation is only on Torah, and I wanted a sample of how he translated this word, in order to spot any trend. In almost all cases, he translated it as gevar, gavra, or guvra. However, six times, in five pesukim, it is translated as anash. And so we should spot the trend for anash and check if that which is unique to anash is absent for gevar. And indeed it is, as you can check below.
The five pesukim are:
person - anash
the word means anyone. It is used together with no. Thus, "there was no person." And this pattern is not found in the many, many instances in Bereishit it is translated as gevar.
The one exception to the pattern is in the last pasuk, where ish occurs twice. The second instance in the pasuk fits the pattern perfectly: no one stood with him when Yosef made himself known to his brothers. But earlier in that same pasuk, anash is used to state "send out every person from me." Perhaps it is the influence of the word later in the pasuk, or because it is a reference to the same individuals, or because "everyone" in certain instances might also merit an anash.
In terms of gevar, it is typically used for ish when talking about a specific person. Noach was a gevar zakkai, an ish tzadik. But also for any general person. It does not really mean everyone but rather each one. Thus, each man should leave his father and mother and cling to his wife, so it is gevar. Each man to his language, so gevar. Each man said to his fellow, so gevar. Inspect the examples of gevar below and see what I mean. The one exception is 41:44, that no man shall lift his hand or foot without Yosef's say-so. Perhaps one can make an argument to distinguish these. But even if Onkelos the human strays in language, which is natural rather than mechanical, I believe that in general the pattern I laid out above emerges.
Here is another example, with one pasuk after the other, with varying translations by Onkelos:
In the first instance, it is one man from his fellow, so the man is selected. In the second instance, there is no man with us, that is that there is not anyone with us, without any particular man selected.
But what then of the instances in parshat Pinchas? Aren't these cases of "anyone"? And I would not readily attribute it to the fluidity of language, for there is also the matter of the shift from one to the other. Of course, anything is possible.
Those pesukim in Pinchas, again, are:
I would suggest that the translation in pasuk 65 of ish as anash should raise no eyebrows, for it is entirely expected. But gevar in pasuk 64 is questionable.
And as a solution, perhaps we can answer that it does not mean "anyone," but rather "each one." It might depend to which word we associate ish, to uv`eleh or to mipekudei. If to uv`eleh, we would expect anash. But if we say, among these there was not -- and what was there not? -- not any individual of the persons who were counted initially. Other suggestions welcome, of course.
The evidence follows, except for anash, which has been listed above already.
man - gevar