OK, I recorded some data after different steps.

Sapphire Pulse RX 5700 XT, LG 27GL85-B (2560x1440, 144 Hz), DisplayPort 1.4
connection enabled in the monitor, DP 1.4 cable used.

1. After normal, boot:

auto

GFX Clocks and Power:
        875 MHz (MCLK)
        800 MHz (SCLK)
        300 MHz (PSTATE_SCLK)
        100 MHz (PSTATE_MCLK)
        750 mV (VDDGFX)
        30.0 W (average GPU)

no flickering.

2. Doing:

echo "high" > /sys/class/drm/card0/device/power_dpm_force_performance_level

GFX Clocks and Power:
        875 MHz (MCLK)
        2045 MHz (SCLK)
        300 MHz (PSTATE_SCLK)
        100 MHz (PSTATE_MCLK)
        1200 mV (VDDGFX)
        33.0 W (average GPU)        

no flickering

3. Doing:

echo "low" > /sys/class/drm/card0/device/power_dpm_force_performance_level

GFX Clocks and Power:
        875 MHz (MCLK)
        300 MHz (SCLK)
        300 MHz (PSTATE_SCLK)
        100 MHz (PSTATE_MCLK)
        750 mV (VDDGFX)
        30.0 W (average GPU)

no flickering

4. Then, before suspending:

echo "auto" > /sys/class/drm/card0/device/power_dpm_force_performance_level

5. Suspend, and resume:

auto

GFX Clocks and Power:
        100 MHz (MCLK)
        800 MHz (SCLK)
        300 MHz (PSTATE_SCLK)
        100 MHz (PSTATE_MCLK)
        750 mV (VDDGFX)
        10.0 W (average GPU)

strong flickering

6. After that doing:

echo 'high' > power_dpm_force_performance_level

GFX Clocks and Power:
        875 MHz (MCLK)
        2045 MHz (SCLK)
        300 MHz (PSTATE_SCLK)
        100 MHz (PSTATE_MCLK)
        1200 mV (VDDGFX)
        33.0 W (average GPU)
no flickering

7. Doing:

echo 'low' > power_dpm_force_performance_level

GFX Clocks and Power:
        100 MHz (MCLK)
        300 MHz (SCLK)
        300 MHz (PSTATE_SCLK)
        100 MHz (PSTATE_MCLK)
        750 mV (VDDGFX)
        10.0 W (average GPU)

no flickering!

8. And then:

echo 'auto' > power_dpm_force_performance_level

GFX Clocks and Power:
        100 MHz (MCLK)
        800 MHz (SCLK)
        300 MHz (PSTATE_SCLK)
        100 MHz (PSTATE_MCLK)
        750 mV (VDDGFX)
        10.0 W (average GPU)

flickering again!