Replacing old dimmer with smart dimmer

I just replaced an old dimmer with a GoControl WD500Z-1. The bulbs installed by the previous home owner are Osram DX1532, the word “dimmable” is not mentioned in the light bulb.

The old dimmer doesn’t dim much, but it does dim smoothly between ~60% and 100%.

The new dimmer doesn’t dim at all.

After replacing one of the 4 bulbs with an incandescent bulb the new dimmer dims the incandescent bulb very well down to 1% and the led bulbs down to ~40%. It looks like the presence of an incandescent light makes the others dimmable.

What’s going on?
Do I need to replace all the led lights with dimmable lights?
Or should I replace only one?

Does the GoControl documentation mention anything about a minimum load?

This page or the linked pdf only mention maximum load, no minimum:

Just in case it helps, here is how it works with 11W led and 60W incandescent:
3x11W+60W=93W: perfect dimming
4x11W=44W: no dimming
3x11W=33W: dimming with horrible flickering
2x11W=22W: bad dimming
11W: so so dimming

I got new led dimmable lights and now it works.

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