I'm here to contribute steps, so you're able to download the P45 in QuickBooks Online.
At times, unusual behavior may occur in QuickBooks. Stored cache files on your browser can sometimes cause this to happen. With this, I'd suggest performing fundamental troubleshooting steps to isolate the dilemma.
Let's begin by accessing your account in incognito. These browsers won't save any browsing histories.
Feel free to use these shortcut keys:
Google Chrome: Ctrl + Shift + N
Safari 11 or newer: ⌘ + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Microsoft Edge: Ctrl + Shift + P
Once you're in, re-download the report to verify if it's already performing accurately. If it's working fine, we'll have to go back to your regular browser and clear the cache to eliminate the junk files. Using other supported browsers is a good alternative too.