That is completely normal. The camera has no idea what the effect of the flash will be and cannot preview it. The effect of flash is extremely variable based on distance and reflectance. For example if you have an object 1m away and one 2m away, the one 2m away gets 4 times less light than the one 1m. Plus, how much it reflects back depends on the object's material and color.

There is no way the camera and similar this without firing the flash which is what modern cameras do before the exposure. This is called TTL Flash which stands for Through The Lens. The camera fires the flash at a low power, measures the results and then adjusts the levels accordingly for the actual shot that follows a fraction of a second later. If you have redeye reduction turned on, the camera will fire 3 times: once to meter, once to contract pupils and once for the exposure.

On some cameras you can fire the pre-flash early and lock in the measured results but I have never seen it update the preview, although technically but if anything in the scene moves, it would no longer be correct. This is called FV-Lock.

For the between the on-board flash and exposure, it depends on the flash mode:

• In Fill mode, the exposure is NOT changed due to flash.
• In Auto Flash mode, exposure is adjusted in consideration based on some measurements of the scene.
• There are other modes (Forced, On, Slow-Sync, Rear-Sync, etc) and how they behave differs between cameras, so read the manual and conduct a few experiments with a static scene.