I use ACR for phone interviews. I've always had my earbuds in during the calls, and ACR has always recorded both sides of the conversation. Yesterday, for the first time, it did not record the person I called. I have not changed any settings. The calls before and after this call recorded both sides of the conversation.
I'm using a Samsung Galaxy S7 running Android version 8.0.0 (Oreo) and Samsung Experience version 9.0. It was updated June 16, 2018.