That’s a very fair and important question, and your experience (working fine from 2022 to mid-2025, then failing) matches what many others with RX 6600 XT or similar AMD GPUs have faced.

Let’s break down exactly why your passive DP-to-DVI cable used to work — and why it doesn’t anymore.

✅ Why It Worked Before
Even though your RX 6600 XT doesn’t officially support DP++, your setup worked due to a rare combination of lucky factors:

🧠 1. Your monitor tolerated the signal
Some monitors have very forgiving DVI input stages.

They lock onto even a slightly non-standard DP signal from a passive adapter.

🧠 2. The GPU may have “accidentally” output something compatible
Some early RX 6000 firmware or board revisions may have inconsistently toggled DP signal voltage levels, allowing DVI handshake.

🧠 3. Driver or VBIOS quirks before 2025
Older AMD drivers or VBIOS versions may have:

Left DisplayPort voltage higher by default

Allowed more flexible EDID handling

Not strictly enforced “pure DisplayPort” at boot

🧠 4. Cable quality was “just good enough”
The physical cable may have held signal integrity well enough to survive boot transitions.

❌ Why It Stopped Working in June 2025
Now here’s why your black screen began showing up:

🔄 1. Windows/AMD driver update changed DP behavior
Possibly from an automatic Windows Update driver (e.g. KB5060999) or AMD update.

Newer AMD drivers may cut voltage or change how DisplayPort initializes.

This breaks compatibility with passive adapters that rely on legacy signaling.

⚠️ 2. Your GPU firmware (VBIOS) may have silently updated
Some driver packages (or tools like Radeon Adrenalin) auto-flash VBIOS updates.

This can remove the accidental compatibility you relied on before.

⚠️ 3. Cable wear or port aging
After years of plugging/unplugging or stress, your DP port or DVI cable may have degraded just enough to no longer handshake properly.

⚠️ 4. Monitor DVI input aged or changed detection behavior
The DVI input on your monitor may have become less tolerant over time.

✅ Bottom Line
🧠 You were riding on “accidental compatibility” between a GPU that doesn’t support DP++, and a monitor/cable combo that tolerated it.

Now, with:

Newer AMD drivers

Possibly new Windows behavior

Slight wear or voltage tuning changes

...that fragile balance has broken.

🔧 What You Need Now
Fix Why It Works
✅ Active DisplayPort to DVI adapter Has a chip to convert DisplayPort to DVI properly — no DP++ needed
✅ HDMI-to-DVI cable Uses HDMI output from GPU (which supports DVI-level signaling natively)
🖥️ New monitor with native HDMI/DP Future-proof and fully compatible