talking to mikroelektronika 2x 4 7-segment display

That's odd - if it's MAX7219 it should work easily as you say.

It's worth trying 5v, however I've run MAX7219 displays off of 3.3v before without problems. Sometimes the default configuration turns all the LEDs on bright which draws a lot of power, but even then I don't think it'd be a big deal on the Pico.

When does the flashing happen? And how fast is it? I'd noticed that happening on MAX7219 before uploading code, as the data lines are floating by default and can input random data.

In the past I'd had problems with that random data setting the 'displayTest' flag inside the MAX7219, but I added code recently to fix that.

You could try setting the SPI baud rate to something lower and seeing if that helps: SPI2.setup({baud:10000,mosi:B15, sck:B13});

You could also try calling displayTest(false), scanLimit(8) and on() just to try and reset values if they had got confused.

However if you're getting flickering even after uploading code, I'd check the connections - in my case flickering was always caused by disconnected data lines (or lines connected to pins that were floating).