There is no bug here. The mobile and desktop views differ in how they display votes, that is all.
On the desktop, new votes are shown as distinct from old posts, and votes in each category are grouped per post, per vote type (downvotes separate from upvotes separate from accepts, etc.). If you have not looked at your reputation tab in a while, new votes for post A are shown grouped separate from new votes for post B, both separate from votes for those same posts you have already 'seen'.
In the by time view the order in which the grouped votes appear is by most-recent vote timestamp; if you received votes for a post at midnight and at noon, the noon vote determines the sort order.
On mobile, all votes are shown in the order they arrived, newest first, and no 'new' highlighting is applied.
However, viewing your votes on mobile resets the 'last viewed' timestamp. As such, viewing your votes on mobile first, then on the desktop, means you won't see those new votes grouped separately anymore.