ArcLight Capital Partners LLC lowered its bid for American Midstream Partners LP, generating fresh backlash from investors days after the Permian pipeline operator scrapped a payout to shareholders.

ArcLight, which indirectly owns American Midstream’s general partner, will now pay $4.50 a unit, according to a statement after the market close Jan. 3. That’s 26 percent lower than its original offer of $6.10 per unit but represents a premium to the Jan. 3 close of $3.15. American Midstream tumbled Dec. 31, 2018, after announcing an amended credit agreement that no longer allowed the company to pay out its fourth-quarter cash distribution....