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Those voltage readings are fine.
An EX250 sitting at idle at a stoplight is the worst voltage situation: not generating a lot of power but having a lot of draw put on the system (headlight, running lights, all brake light bulbs at their "high" 2-filament setting because you're holding the brake "on", maybe even a turn signal flashing...). EX250s all do that dimming thing under these conditions.
As to the instrument lighting, if you've got just one single little light that's intermittent I'd bet on it being a bad bulb (one that is somehow not up to the normal quality standard you find in motorcycles, they normally last many years). In the case of just a single light bulb going intermittent or quitting completely I'd get a few replacement bulbs (I always replace old motorcycle instrument bulbs with more modern LED versions). Although it could be the problem bulb simply isn't seated properly or secured properly in its holder.
If you have a whole area of the instrument panel going dark (like the entire tachometer gauge which probably has 3 or 4 small bulbs around/behind it), meaning you've got several bulbs going dim or quitting all at once (hard to tell what's actually happening from your description) I'd be checking the power and grounding connections on the backside of the instrument cluster.
If you remove the surrounding fairing/windshield and then loosen the instrument cluster nuts you'll be able to access the backside of the cluster. You'll see that the lighting is really pretty simple.
If this bike is always parked outside in the weather there could be some corrosion developing in some of the connector plugs. Any connector plug that regularly gets wet can corrode, but especially any connectors that see water/dampness plus high voltage current will be the first to corrode, the worst tends to be the 3-wire (3 yellow wires) connector plug that carries the 3 phases of AC power generated by the rotor/stator to the regulator/rectifier.
And, it doesn't matter how much power your rotor/stator is generating if the bike's main ground is messed up or loose. The bolt that secures the main electrical system ground wire needs to be clean and properly torqued. The threads in the engine block that this bolt screws into also needs to be clean for proper grounding. If you don't know where this ground is located just follow the big black wire from the battery negative pole, it will lead you right to the main ground.
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