It's this way with many prepositions - אל + הם = אליהם /ele(y)hem/, על + הם = עליהם /ele(y)hem/. Yet other prepositions do אצל+ הם = אצלם (/etslam/), עם + הם = עימם /imam/. My guess: for ancient Hebrew speakers it was phonetically hard to follow the closed syllable with a consonant ה, so they swiped a vowel in between. Supporting evidence: prepositions that end with vowel do attach הם as-is: לפיהם, להם.
the English is correct, they've been marking things wrong for tiny errors with the new update, and other times marking things correct that I accidentally mis-swyped (mistyped), so I'd chalk it up to a mistype unless it happens again. (Sorry I can't see when you left this comment, I'm in the app).
24 April 2019
Well, no, יְשׁוֹבֵב is the imperfect of the Polel שׁוֹבֵב formed from the root שוב to return, meaning to bring back, to restore, refresh. נפשי יושבב translates to he restores my soul. So no similarity at all. It is nowadays only used in a lofty style: מְשׁוֹבֵב נֶ֫פֶשׁ לִראוֹתְךָ בְּחַיּיִם, יְדִידִי it lifts spirits to see you alive, my friend or אַתְּ מַרְאֶה מְשׁוֹבֵב נֶ֫פֶשׁ you are a sight for sour eyes.