the FD struggles to follow the GS also with VNAV disengaged.
TRY THIS:
start at KSFO rwy28-R with no flight plan loaded. Take off, right 180 degrees turn, climb at 3000ft and set AP (HDG mode). Fly straight ahead until 12 miles DME from IGWQ loc-ils (111.7) Then turn right again to intercept LOC and arm APP. With correct approach configuration (speed,gear,flaps), the ATR will correctly intercept LOC and GS (LOC/GS green)but at a certain point U ll see the vertical GS green diamond slowly beginning to raise and FD not commanding a climb to recover it.
Try the same procedure with the TBM930 and U ll see the aircraft perfectly stable on both GS and LOC until runway threshold.
ALL the vertical autopilot modes in this aircraft (VNAV AND GS follwing) need some tuning in my opinion😉