Sounds like someone was trying to make inverted flight rather easy.

Here are some aerodynamic truths:

1. The Horizontal Stabilizer IS the active incidence setter. The longitudinal axis is simply a designer's / drawer's / or whomever's working line. The stabilizer sets and holds the required aerodynamic realities.

2. In sub-sonic convergent airflow, Given a symetrical stabilizer, and a symetrical wing, any real incidence is the trim of the stabilizer to hold the wing in a flying position. There are numerous examples. If you have a lifting wing, stab. elevator trim/position loads the wing to stay where you want it. Flaps down changes the wing Angle of Attack (AOA) which changes the lift. Lift is a result of basically Air Mass Density, Square of the Airspeed (flow over/under the wing) and surface area.
Changes require additional change.
#1. 0° Stabilizer reference the longitudinal axis: Same for wing: Then the sym. wing must be forced into a lifting position (Positive angle-of-attack) by trim of the stab. Given then the stab/elevator is set to something "up"to hold the wing into a lifting position. If the engine thrust line is also 0 to the axis, and is now inline with the wing chord line there is another force to be contained by stab trim.
2. The stabilizer is nothing more or less than a tool to hold the wing in a loaded position. Engine thrust is in attempt to pull the machine in the direction of the thrust-line. Most single-engine prop airplanes have a vertical stabilizer that is offset a couple +/- degrees leading edge to left to assist with the torque problem during high power operations.

To make a LOOOONNGGG story short, try to rebuild your machine to have your thrust-line, wing center line (LE to TE) aligned with the stab line. Many of the older Pattern RC Models were designed with stab and wing in alignment. Then the engine was set at 1.5 to 2° down thrust. This meant that the engine pulled down, causing the wing to be generating some lift positive, the elevator trimmed a tad up to hold the wing in a positive Angle of Attack(AOA) to be providing lift, then when the machine is turned up-side down, the engine is pulling up while the elevator is wanting the wing to go down. The pilot's punches in a tad of down-trim, (Now UP ) to assist the wing to be in a lifting AOA.

One can fly a model which is way out of reality aerodynamic rules. The 3-d folks do it all day every day. OTOH I am a sport flier mostly as I did Pylon, competition
Fun Fly, and tried pattern but not my style. Scale (somewhat-scale that is ) is my forte. Enjoy your pleasure but a small understanding about flying machines makes things easier.