This is usually for a good reason - dual sim phones are almost always “DSDS”, or “Dual SIM Dual Standby”. The secondary SIM, because it doesn’t need to make a data connection, parks itself on the lowest-frequency (and therefore usually lowest-bandwidth) connection it can find. Meanwhile, your data-connected SIM is busy trying to stream a video or upload your photos, so it’s using a higher-frequency + higher bandwidth connection, resulting in a lower signal strength.

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dataflow 4 days ago | parent [-]

> Meanwhile, your data-connected SIM is busy trying to stream a video or upload your photos

You're making huge and incorrect assumptions here, no? This also happens when your phone is entirely idle... and it randomly changes if you sit still for some time...

	▲	estimator7292 3 days ago \| parent \| next [-]
		Your phone is never idle. Open your adb logs sometime and you'll see that it's doing a ton of stuff all the time. Much of which involves a network connection. So your phone is basically always doing something, and frequently sending and receiving data when you assume it's doing nothing. By design, radios hop around between channels as conditions change. Another device somewhere outside your house kicked off a big transfer and your device hopper channels to avoid interference. Random atmospheric conditions introduced new noise, or another channel cleared up. This is standard, normal behavior for WiFi, Bluetooth, cellular, and essentially every other type of modern digital radio. What you're seeing is normal and expected behavior. Modern radios and operating systems are vastly more complex than you're assuming.
	▲	lxgr 4 days ago \| parent \| prev [-]
		How do you know that your phone is entirely idle?