UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q ☒ Quarterly Report Pursuant to Section 13 or 15(d) of The Securities Exchange Act of 1934 For the quarterly period ended September 30, 2025 Isabella Bank Corporation (Exact name of registrant as specified in its charter) (State or other jurisdiction of incorporation or organization) (I.R.S. Employer Identification No.) 401 N. Main St Mt. Pleasant MI 48858 (Address of principal executive offices) (Zip code) or (989) 772-9471 (R ...