If you disable GPU_texture_update_sub when there is more than one row, then you're effectively disabling it entirely? Since I guess you usually need more than one row.

It's usually just one row.
Multiple rows only appear when typing many different characters or maximizing the scale of the text.

Ok, but then I do wonder if this will actually solve T74228: strange characters? Since I see no indication that is a case where it would need multiple rows.

In D6936#164895, @Brecht Van Lommel (brecht) wrote:

Ok, but then I do wonder if this will actually solve T74228: strange characters? Since I see no indication that is a case where it would need multiple rows.

There some characters are working and others are not.
Those that are not working seem to be the result of a poor alignment of the buffer within the texture (reading characters in the wrong position and with different size).
It is possible that the problem is in the shader, but it is more likely to be in the part indicated here.