Does the glide slope flag detect percent of modulation (80%)?
I wanted to post this as a comment, but it's a bit long.
From ICAO Chicago Convention annex 10, vol 1:

2.3 Alarm conditions for ILS airborne equipment
2.3.1 Ideally, a receiver alarm system such as a visual flag should warn a pilot
of any unacceptable malfunctioning conditions which might
arise within either the ground or airborne equipments. The extent to
which such an ideal may be satisfied is specified below.
...
2.3.3 The alarm system should indicate to the pilot and to any other airborne system which may be utilizing the localizer and glide path
data, the existence of any of the following conditions:
...
b) the percentage modulation of either the 90 Hz or 150 Hz signal
reduction to zero with the other maintained at its normal 20 per cent
and 40 per cent modulation respectively for the localizer and glide
path;
Note.— It is expected that the localizer alarm occur when either the
90 Hz or 150 Hz modulation is reduced to 10 per cent with the other
maintained at its normal 20 per cent. It is expected that the glide
path alarm occur when either the 90 Hz or 150 Hz modulation is reduced
to 20 per cent with the other maintained at its normal 40 per cent.

Glide path receiving equipment operational standards are detailed in RTCA DO-192 and EUROCAE ED-47B