The root אשר seems to act weirdly in relation to number.
On the one hand, it takes on plural morphology in situations that are clearly singular, here in the form of a plural (const.) noun (functioning as an adj.) in reference to a sing. noun: אַשְׁרֵי הָאִישׁ אֲשֶׁר לֹא הָלַךְ בַּעֲצַת רְשָׁעִים (Ps. 1:1)
On the other hand, inversely, we sing as we return the sefer Torah to the ark, of a singular adj. qualifying a plural noun: עץ חיים היא למחזיקים בה ותמכיה מְאֻשָּׁר(Prov. 3:18)
Finally, there are cases that appear altogether consistent, here plural with plural: אַשְׁרֵי אֲנָשֶׁיךָ אַשְׁרֵי עֲבָדֶיךָ (I Kings 10:8)
Is this just an anomaly of the root אשר, or is something else going on?