PMDG 737-900 slowly looses glideslope on final approach

[gugugs 0]

Yesterday I flew with the PMDG 737-900 from EDDM to EDDP. The localizer was successfully activated during the approach. Then successfully caught the glideslope. Frequencies were set correctly on both NAVs. The two course selectors were also set correctly. The ILS was also displayed in the PFD. Both CMDs were switched on. VREF was also set correctly to 30/150, so overall everything was in the green zone.

After the glideslope was held correctly up to about 500ft AGL, the aircraft suddenly started to level off and slowly but surely left the glideslope. The course was still held. The glideslope was not left frantically or anything. It just very slowly stopped holding it. I then switched off the autopilot and was just able to land manually.

What I also noticed beforehand was that at some point after the ILS was established, a small yellow text box appeared below the FMA and to the right of the ILS display in the PFD. Unfortunately, I couldn't read exactly what it said and it went away again after a few seconds.

I can't understand why this happened. I have now flown at least 50 approaches with the PMDG 737-900 and this has never happened before.

/edit: Could it be that I was simply going too fast?
How does the PMDG 737 react if the selected speed on the MCP is too far away from IAS? Is the speed then accepted and the glideslope maintained, or does the autopilot automatically leave the glideslope at some point to reduce the speed?

 

Regards

Lukas

Edited January 24 by gugugs