CRJ900 Captain reported not noticing auto pilot disconnected from glideslope while descending resulted in a low altitude alert.

Narrative

Coming in to ZZZ on the visual backed by ILS for Runway XX ATC assigned us a speed of 180 until the FAF this vectored us too close to the aircraft we were following. On the decent from the FAF (on glide slope) tower approved us for 1 S turn. The PF (Pilot Flying) performed the S turn using HDG mode and Auto Pilot on. We (the pilots) were not aware that the aircraft had disengaged from the GS but was continuing its vertical descent. The aircraft was still following the Flight Director and visually we had looked only slightly low (3 red on PAPI); we were already beginning to correct when we received the Low Alt alert. I don't remember what our altitude in relation was to the runway at that time. The PF quickly disconnected autopilot and corrected altitude and course. Since it was a visual approach and the aircraft was already fully configured and stable we made the decision to continue the approach and land the aircraft. There was no further incidents. Cause: S turn followed by loss of situational awareness as well as pilot and possible aircraft error. The aircraft was reporting a stable descent on the GS but it was likely not accurate since we had to perform an S turn in HDG mode and were not on the localizer. Suggestion: PF disconnect autopilot and hand fly a visual approach. PM (Pilot Monitoring) Read and report altitudes as on the approach as well as suggest appropriate corrections.