Thank you for reaching out and sorry for the delay in response.
Adobe Acrobat was always 32 bit for Windows machines. There has nothing changed, only after the recent update 32-bit started appearing along with the application name. Acrobat Pro DC 64 bit is currently not available for your geographical region. For more information about this, please refer to the following help document: https://helpx.adobe.com/acrobat/using/whats-new.html#AcrobatDCdesktopapp.
As you are experiencing an issue with combining files, please confirm if the file is saved locally on the machine.
Do you experience this issue only with the current set of files?
Please share the Acrobat and OS version on the machine.
Try combining files online and check if that works fine. Open the link https://documentcloud.adobe.com/ and sign in with your Adobe ID. Then go to Edit > Combine Files.