Hi
this is quite "normal" with the Hobbywing 130A v4 (and other HWs too)... i've seen those voltage dips with Rex3, Rex6+7A, jLog, Spektrum TM1000, etc over many years... and of course with Spirit RS and Spirit GTR units too. how deep it goes (on fbl side) is depending on the sample frequency the different units are using to measure the voltage and the (built-in) caps they are using to stabilise voltage. and of course the electronics attached and/or built in. your gtr has i.e. a complete Jeti duplex telemetry receiver integrated (which is not the case with your other unit). and as you say, a gps attached.
setup of course is playing another role... the more aggressive/demanding the servos are used, the lower the dips. so 2 units cannot be really compared easily. and the question is: is your other unit measuring voltage with own sensor or using esc telemetry values - from the other side of the long cable
the dips i've measured (with dso, channel #1 on the fbl units and channel #2 on the esc bec output) are strongly related to bec cable quality, length and bec connector quality. they are almost not existent at esc side (and thus in esc bec voltage telemetry) and high at fbl side where the power is used. i've i.e. replaced both stock cables of my Hobbywing 130a v4 in a Diabolo 700 with high quality ones ~0.5mm^2 (muldenthal,power box,etc in my case self crimped) and the voltage dips measured now are not from 7.7V down to 7.0V (and sometimes below) anymore, but only down to 7.4-7.5V max. => perfect 10-15 bucks solution to rx voltage dips.
Ciao
Michael
nb: btw i did not experience this huge difference with i.e. YGE Aureus in my 2nd Diabolo 700 with same Spirit RS (now GTR) and same servos... this was what made me curious to measure. as it showed, main reason was bec cable/connector quality in my case.
