I have X-Plane 12.08rc3 with the RXP GTN 650 and GTN 750 installed.  RXP GTN version 2.6.1.1 (update info shows 2.6.1.0 but I installed 2.6.1.1 ...).  On RNAV approaches with vertical guidance (LPV, LNAV/VNAV etc) the aircraft HSI shows "NO GP" and vertical DI info is not shown.  Problem demonstrated with the LR Baron 58 + GTN 750 and LR Cessna 172 G1000 + GTN 650.  Laminar Research tells me the issue is 12.08 introduced a new dataref: sim/cockpit/radios/glideslope_signal_valid[4], which needs to be set to 1 for GPS approaches with vertical guidance.  I verified this with a LUA script.  I hope RXP can provide a fix in an upcoming release.

I am having the same problem. Is there a way for me to correct this problem while we wait for RXP to make an update? Im not sure what LUA script is. 

I'm also seeing no GPS glide path during LPV approaches using RXP GTN Touch 2.7.10 in XP v12.0.9-rc5 an Aerobask Legacy RG v12.1.r1 (latest versions of all three).  ILS approach GS CDI appears normally.   I tried ticking ON the RXP GTN Advanced Settings "Use Simulator GPS Commands" but it made no difference.   Gather from HeinzXP's post that this has been seen since XP v12.08 in both LR and now aerobask a/c.  Perhaps caused by a change in dataref, presumably affecting both LR and 3rd party (e.g. Aerobask) aircraft.    Is a fix possible from the RXP side, so we can fly LPVs?