I'm glad you've reached out to the Community. Let me help resolve this issue so you're able to Prepare GST/HST return with the exact date and amount in QuickBooks Online (QBO).
I've replicated this on my QBO sample file, changing the date range and filing period will automatically adjust the tax amount on my sales tax return.
To check if it caused by the browser, let's open your browser in incognito/private mode. This mode doesn't use the existing cache data files to load a webpage.
Here are the keyboard shortcuts to open a private browser:
Google Chrome: press Ctrl+Shift+N
Mozilla Firefox: press Ctrl+Shift+P
Internet Explorer: press Ctrl+Shift+P
Safari: press Command+Shift+N
If it works fine, I suggest clearing your browser's cache to start fresh. The next time you log in to QuickBooks Online, your browser will download fresh copies of everything you see on each page. Any cache-related issues should be cleared up.
If the same thing happens, I recommend contacting our QuickBooks Online Support Team. They have additional tools to pull up your account and do a screen-sharing to investigate this further.