An ABR is generally going to make an estimate based on observed bandwidth and select an appropriate bitrate for that. It's not out of the question that you run out of forward buffer when your bandwidth takes a nosedive, because the high bitrate video is heavy as all hell and the ABR needs to have observed the drop in bandwidth before it reconsiders and selects a lower bitrate track.

I'm not familiar with ABRs affecting the size of the forward buffer, most commonly these are tweaked based on the type of use-case and scaled in seconds of media.